diff --git a/src/Mod/Assembly/App/AssemblyObject.cpp b/src/Mod/Assembly/App/AssemblyObject.cpp
index 858e0354dd..a15ad2bd61 100644
--- a/src/Mod/Assembly/App/AssemblyObject.cpp
+++ b/src/Mod/Assembly/App/AssemblyObject.cpp
@@ -257,6 +257,10 @@ void AssemblyObject::doDragStep()
 
 Base::Placement AssemblyObject::getMbdPlacement(std::shared_ptr<ASMTPart> mbdPart)
 {
+    if (!mbdPart) {
+        return Base::Placement();
+    }
+
     double x, y, z;
     mbdPart->getPosition3D(x, y, z);
     Base::Vector3d pos = Base::Vector3d(x, y, z);
@@ -407,12 +411,19 @@ void AssemblyObject::redrawJointPlacements(std::vector<App::DocumentObject*> joi
 {
     // Notify the joint objects that the transform of the coin object changed.
     for (auto* joint : joints) {
+        if (!joint) {
+            continue;
+        }
         redrawJointPlacement(joint);
     }
 }
 
 void AssemblyObject::redrawJointPlacement(App::DocumentObject* joint)
 {
+    if (!joint) {
+        return;
+    }
+
     // Notify the joint object that the transform of the coin object changed.
     auto* pPlc = dynamic_cast<App::PropertyPlacement*>(joint->getPropertyByName("Placement1"));
     if (pPlc) {
@@ -429,6 +440,10 @@ void AssemblyObject::recomputeJointPlacements(std::vector<App::DocumentObject*>
 {
     // The Placement1 and Placement2 of each joint needs to be updated as the parts moved.
     for (auto* joint : joints) {
+        if (!joint) {
+            continue;
+        }
+
         App::PropertyPythonObject* proxy = joint
             ? dynamic_cast<App::PropertyPythonObject*>(joint->getPropertyByName("Proxy"))
             : nullptr;
@@ -467,6 +482,10 @@ std::shared_ptr<ASMTAssembly> AssemblyObject::makeMbdAssembly()
 App::DocumentObject* AssemblyObject::getJointOfPartConnectingToGround(App::DocumentObject* part,
                                                                       std::string& name)
 {
+    if (!part) {
+        return nullptr;
+    }
+
     std::vector<App::DocumentObject*> joints = getJointsOfPart(part);
 
     for (auto joint : joints) {
@@ -494,6 +513,10 @@ App::DocumentObject* AssemblyObject::getJointOfPartConnectingToGround(App::Docum
 
 JointGroup* AssemblyObject::getJointGroup(const App::Part* part)
 {
+    if (!part) {
+        return nullptr;
+    }
+
     App::Document* doc = part->getDocument();
 
     std::vector<DocumentObject*> jointGroups =
@@ -614,6 +637,10 @@ std::vector<App::DocumentObject*> AssemblyObject::getGroundedJoints()
 
 std::vector<App::DocumentObject*> AssemblyObject::getJointsOfObj(App::DocumentObject* obj)
 {
+    if (!obj) {
+        return {};
+    }
+
     std::vector<App::DocumentObject*> joints = getJoints(false);
     std::vector<App::DocumentObject*> jointsOf;
 
@@ -630,6 +657,10 @@ std::vector<App::DocumentObject*> AssemblyObject::getJointsOfObj(App::DocumentOb
 
 std::vector<App::DocumentObject*> AssemblyObject::getJointsOfPart(App::DocumentObject* part)
 {
+    if (!part) {
+        return {};
+    }
+
     std::vector<App::DocumentObject*> joints = getJoints(false);
     std::vector<App::DocumentObject*> jointsOf;
 
@@ -693,6 +724,10 @@ void AssemblyObject::fixGroundedPart(App::DocumentObject* obj,
                                      Base::Placement& plc,
                                      std::string& name)
 {
+    if (!obj) {
+        return;
+    }
+
     std::string markerName1 = "marker-" + obj->getFullName();
     auto mbdMarker1 = makeMbdMarker(markerName1, plc);
     mbdAssembly->addMarker(mbdMarker1);
@@ -717,11 +752,14 @@ void AssemblyObject::fixGroundedPart(App::DocumentObject* obj,
 
 bool AssemblyObject::isJointConnectingPartToGround(App::DocumentObject* joint, const char* propname)
 {
-    if (!isJointTypeConnecting(joint)) {
+    if (!joint || !isJointTypeConnecting(joint)) {
         return false;
     }
 
     App::DocumentObject* part = getMovingPartFromRef(joint, propname);
+    if (!part) {
+        return false;
+    }
 
     // Check if the part is grounded.
     bool isGrounded = isPartGrounded(part);
@@ -765,6 +803,10 @@ bool AssemblyObject::isJointConnectingPartToGround(App::DocumentObject* joint, c
 
 bool AssemblyObject::isJointTypeConnecting(App::DocumentObject* joint)
 {
+    if (!joint) {
+        return false;
+    }
+
     JointType jointType = getJointType(joint);
     return jointType != JointType::RackPinion && jointType != JointType::Screw
         && jointType != JointType::Gears && jointType != JointType::Belt;
@@ -773,6 +815,10 @@ bool AssemblyObject::isJointTypeConnecting(App::DocumentObject* joint)
 
 bool AssemblyObject::isObjInSetOfObjRefs(App::DocumentObject* obj, const std::vector<ObjRef>& set)
 {
+    if (!obj) {
+        return false;
+    }
+
     for (const auto& pair : set) {
         if (pair.obj == obj) {
             return true;
@@ -835,7 +881,12 @@ std::vector<ObjRef>
 AssemblyObject::getConnectedParts(App::DocumentObject* part,
                                   const std::vector<App::DocumentObject*>& joints)
 {
+    if (!part) {
+        return {};
+    }
+
     std::vector<ObjRef> connectedParts;
+
     for (auto joint : joints) {
         if (!isJointTypeConnecting(joint)) {
             continue;
@@ -865,6 +916,10 @@ AssemblyObject::getConnectedParts(App::DocumentObject* part,
 
 bool AssemblyObject::isPartGrounded(App::DocumentObject* obj)
 {
+    if (!obj) {
+        return false;
+    }
+
     std::vector<App::DocumentObject*> groundedObjs = getGroundedParts();
 
     for (auto* groundedObj : groundedObjs) {
@@ -878,6 +933,10 @@ bool AssemblyObject::isPartGrounded(App::DocumentObject* obj)
 
 bool AssemblyObject::isPartConnected(App::DocumentObject* obj)
 {
+    if (!obj) {
+        return false;
+    }
+
     std::vector<App::DocumentObject*> groundedObjs = getGroundedParts();
     std::vector<App::DocumentObject*> joints = getJoints(false);
 
@@ -1186,6 +1245,10 @@ std::shared_ptr<ASMTJoint> AssemblyObject::makeMbdJointDistance(App::DocumentObj
 std::vector<std::shared_ptr<MbD::ASMTJoint>>
 AssemblyObject::makeMbdJoint(App::DocumentObject* joint)
 {
+    if (!joint) {
+        return {};
+    }
+
     JointType jointType = getJointType(joint);
 
     std::shared_ptr<ASMTJoint> mbdJoint = makeMbdJointOfType(joint, jointType);
@@ -1547,6 +1610,10 @@ AssemblyObject::MbDPartData AssemblyObject::getMbDData(App::DocumentObject* part
 
 std::shared_ptr<ASMTPart> AssemblyObject::getMbDPart(App::DocumentObject* part)
 {
+    if (!part) {
+        return nullptr;
+    }
+
     return getMbDData(part).part;
 }
 
@@ -1606,6 +1673,10 @@ std::shared_ptr<ASMTMarker> AssemblyObject::makeMbdMarker(std::string& name, Bas
 std::vector<ObjRef> AssemblyObject::getDownstreamParts(App::DocumentObject* part,
                                                        App::DocumentObject* joint)
 {
+    if (!part) {
+        return {};
+    }
+
     // First we deactivate the joint
     bool state = false;
     if (joint) {
@@ -1628,59 +1699,17 @@ std::vector<ObjRef> AssemblyObject::getDownstreamParts(App::DocumentObject* part
     if (joint) {
         AssemblyObject::setJointActivated(joint, state);
     }
-    /*if (limit > 1000) {  // Infinite loop protection
-        return {};
-    }
-    limit++;
-    Base::Console().Warning("limit %d\n", limit);
 
-    std::vector<App::DocumentObject*> downstreamParts = {part};
-    std::string name;
-    App::DocumentObject* connectingJoint =
-        getJointOfPartConnectingToGround(part,
-                                         name);  // ?????????????????????????????? if we remove
-                                                 // connection to ground then it can't work for tom
-    std::vector<App::DocumentObject*> jointsOfPart = getJointsOfPart(part);
-
-    // remove connectingJoint from jointsOfPart
-    auto it = std::remove(jointsOfPart.begin(), jointsOfPart.end(), connectingJoint);
-    jointsOfPart.erase(it, jointsOfPart.end());
-    for (auto joint : jointsOfPart) {
-        App::DocumentObject* part1 = getMovingPartFromRef(joint, "Reference1");
-        App::DocumentObject* part2 = getMovingPartFromRef(joint, "Reference2");
-        bool firstIsDown = part->getFullName() == part2->getFullName();
-        App::DocumentObject* downstreamPart = firstIsDown ? part1 : part2;
-
-        Base::Console().Warning("looping\n");
-        // it is possible that the part is connected to ground by this joint.
-        // In which case we should not select those parts. To test we disconnect :
-        auto* propObj = dynamic_cast<App::PropertyLink*>(joint->getPropertyByName("Part1"));
-        if (!propObj) {
-            continue;
-        }
-        propObj->setValue(nullptr);
-        bool isConnected = isPartConnected(downstreamPart);
-        propObj->setValue(part1);
-        if (isConnected) {
-            Base::Console().Warning("continue\n");
-            continue;
-        }
-
-        std::vector<App::DocumentObject*> subDownstreamParts =
-            getDownstreamParts(downstreamPart, limit);
-        for (auto downPart : subDownstreamParts) {
-            if (std::find(downstreamParts.begin(), downstreamParts.end(), downPart)
-                == downstreamParts.end()) {
-                downstreamParts.push_back(downPart);
-            }
-        }
-    }*/
     return downstreamParts;
 }
 
 std::vector<App::DocumentObject*> AssemblyObject::getUpstreamParts(App::DocumentObject* part,
                                                                    int limit)
 {
+    if (!part) {
+        return {};
+    }
+
     if (limit > 1000) {  // Infinite loop protection
         return {};
     }
@@ -1704,7 +1733,7 @@ App::DocumentObject* AssemblyObject::getUpstreamMovingPart(App::DocumentObject*
                                                            App::DocumentObject*& joint,
                                                            std::string& name)
 {
-    if (isPartGrounded(part)) {
+    if (!part || isPartGrounded(part)) {
         return nullptr;
     }
 
@@ -1721,6 +1750,10 @@ App::DocumentObject* AssemblyObject::getUpstreamMovingPart(App::DocumentObject*
 
 double AssemblyObject::getObjMass(App::DocumentObject* obj)
 {
+    if (!obj) {
+        return 0.0;
+    }
+
     for (auto& pair : objMasses) {
         if (pair.first == obj) {
             return pair.second;
@@ -1809,6 +1842,10 @@ void AssemblyObject::ensureIdentityPlacements()
 
 void AssemblyObject::swapJCS(App::DocumentObject* joint)
 {
+    if (!joint) {
+        return;
+    }
+
     auto pPlc1 = dynamic_cast<App::PropertyPlacement*>(joint->getPropertyByName("Placement1"));
     auto pPlc2 = dynamic_cast<App::PropertyPlacement*>(joint->getPropertyByName("Placement2"));
     if (pPlc1 && pPlc2) {
@@ -1833,7 +1870,11 @@ bool AssemblyObject::isEdgeType(App::DocumentObject* obj,
                                 std::string& elName,
                                 GeomAbs_CurveType type)
 {
-    PartApp::Feature* base = static_cast<PartApp::Feature*>(obj);
+    auto* base = dynamic_cast<PartApp::Feature*>(obj);
+    if (!base) {
+        return false;
+    }
+
     const PartApp::TopoShape& TopShape = base->Shape.getShape();
 
     // Check for valid face types
@@ -1851,8 +1892,12 @@ bool AssemblyObject::isFaceType(App::DocumentObject* obj,
                                 std::string& elName,
                                 GeomAbs_SurfaceType type)
 {
-    auto base = static_cast<PartApp::Feature*>(obj);
-    PartApp::TopoShape TopShape = base->Shape.getShape();
+    auto* base = dynamic_cast<PartApp::Feature*>(obj);
+    if (!base) {
+        return false;
+    }
+
+    const PartApp::TopoShape TopShape = base->Shape.getShape();
 
     // Check for valid face types
     TopoDS_Face face = TopoDS::Face(TopShape.getSubShape(elName.c_str()));
@@ -1867,7 +1912,11 @@ bool AssemblyObject::isFaceType(App::DocumentObject* obj,
 
 double AssemblyObject::getFaceRadius(App::DocumentObject* obj, std::string& elt)
 {
-    auto base = static_cast<PartApp::Feature*>(obj);
+    auto* base = dynamic_cast<PartApp::Feature*>(obj);
+    if (!base) {
+        return 0.0;
+    }
+
     const PartApp::TopoShape& TopShape = base->Shape.getShape();
 
     // Check for valid face types
@@ -1886,7 +1935,11 @@ double AssemblyObject::getFaceRadius(App::DocumentObject* obj, std::string& elt)
 
 double AssemblyObject::getEdgeRadius(App::DocumentObject* obj, std::string& elt)
 {
-    auto base = static_cast<PartApp::Feature*>(obj);
+    auto* base = dynamic_cast<PartApp::Feature*>(obj);
+    if (!base) {
+        return 0.0;
+    }
+
     const PartApp::TopoShape& TopShape = base->Shape.getShape();
 
     // Check for valid face types
@@ -1902,6 +1955,10 @@ double AssemblyObject::getEdgeRadius(App::DocumentObject* obj, std::string& elt)
 
 DistanceType AssemblyObject::getDistanceType(App::DocumentObject* joint)
 {
+    if (!joint) {
+        return DistanceType::Other;
+    }
+
     std::string type1 = getElementTypeFromProp(joint, "Reference1");
     std::string type2 = getElementTypeFromProp(joint, "Reference2");
     std::string elt1 = getElementFromProp(joint, "Reference1");
@@ -2119,6 +2176,10 @@ DistanceType AssemblyObject::getDistanceType(App::DocumentObject* joint)
 
 void AssemblyObject::setJointActivated(App::DocumentObject* joint, bool val)
 {
+    if (!joint) {
+        return;
+    }
+
     auto* propActivated = dynamic_cast<App::PropertyBool*>(joint->getPropertyByName("Activated"));
     if (propActivated) {
         propActivated->setValue(val);
@@ -2127,6 +2188,10 @@ void AssemblyObject::setJointActivated(App::DocumentObject* joint, bool val)
 
 bool AssemblyObject::getJointActivated(App::DocumentObject* joint)
 {
+    if (!joint) {
+        return false;
+    }
+
     auto* propActivated = dynamic_cast<App::PropertyBool*>(joint->getPropertyByName("Activated"));
     if (propActivated) {
         return propActivated->getValue();
@@ -2137,6 +2202,9 @@ bool AssemblyObject::getJointActivated(App::DocumentObject* joint)
 double AssemblyObject::getJointDistance(App::DocumentObject* joint)
 {
     double distance = 0.0;
+    if (!joint) {
+        return distance;
+    }
 
     auto* prop = dynamic_cast<App::PropertyFloat*>(joint->getPropertyByName("Distance"));
     if (prop) {
@@ -2149,6 +2217,9 @@ double AssemblyObject::getJointDistance(App::DocumentObject* joint)
 double AssemblyObject::getJointDistance2(App::DocumentObject* joint)
 {
     double distance = 0.0;
+    if (!joint) {
+        return distance;
+    }
 
     auto* prop = dynamic_cast<App::PropertyFloat*>(joint->getPropertyByName("Distance2"));
     if (prop) {
@@ -2161,6 +2232,9 @@ double AssemblyObject::getJointDistance2(App::DocumentObject* joint)
 JointType AssemblyObject::getJointType(App::DocumentObject* joint)
 {
     JointType jointType = JointType::Fixed;
+    if (!joint) {
+        return jointType;
+    }
 
     auto* prop = dynamic_cast<App::PropertyEnumeration*>(joint->getPropertyByName("JointType"));
     if (prop) {
@@ -2186,6 +2260,10 @@ std::vector<std::string> AssemblyObject::getSubAsList(App::PropertyXLinkSub* pro
 
 std::vector<std::string> AssemblyObject::getSubAsList(App::DocumentObject* obj, const char* pName)
 {
+    if (!obj) {
+        return {};
+    }
+
     auto* prop = dynamic_cast<App::PropertyXLinkSub*>(obj->getPropertyByName(pName));
 
     return getSubAsList(prop);
@@ -2193,6 +2271,10 @@ std::vector<std::string> AssemblyObject::getSubAsList(App::DocumentObject* obj,
 
 std::string AssemblyObject::getElementFromProp(App::DocumentObject* obj, const char* pName)
 {
+    if (!obj) {
+        return "";
+    }
+
     std::vector<std::string> names = getSubAsList(obj, pName);
 
     if (names.empty()) {
@@ -2216,6 +2298,10 @@ std::string AssemblyObject::getElementTypeFromProp(App::DocumentObject* obj, con
 
 App::DocumentObject* AssemblyObject::getObjFromProp(App::DocumentObject* joint, const char* pName)
 {
+    if (!joint) {
+        return nullptr;
+    }
+
     auto* propObj = dynamic_cast<App::PropertyLink*>(joint->getPropertyByName(pName));
     if (!propObj) {
         return nullptr;
@@ -2323,6 +2409,10 @@ App::DocumentObject* AssemblyObject::getObjFromRef(App::PropertyXLinkSub* prop)
 
 App::DocumentObject* AssemblyObject::getObjFromRef(App::DocumentObject* joint, const char* pName)
 {
+    if (!joint) {
+        return nullptr;
+    }
+
     auto* prop = dynamic_cast<App::PropertyXLinkSub*>(joint->getPropertyByName(pName));
 
     return getObjFromRef(prop);
@@ -2397,6 +2487,10 @@ App::DocumentObject* AssemblyObject::getMovingPartFromRef(App::PropertyXLinkSub*
 App::DocumentObject* AssemblyObject::getMovingPartFromRef(App::DocumentObject* joint,
                                                           const char* pName)
 {
+    if (!joint) {
+        return nullptr;
+    }
+
     auto* prop = dynamic_cast<App::PropertyXLinkSub*>(joint->getPropertyByName(pName));
 
     return getMovingPartFromRef(prop);
@@ -2405,6 +2499,10 @@ App::DocumentObject* AssemblyObject::getMovingPartFromRef(App::DocumentObject* j
 App::DocumentObject* AssemblyObject::getLinkedObjFromRef(App::DocumentObject* joint,
                                                          const char* pObj)
 {
+    if (!joint) {
+        return nullptr;
+    }
+
     auto* obj = getObjFromRef(joint, pObj);
     if (obj) {
         return obj->getLinkedObject(true);