Assembly: Prevent crash when link to linkgroup in subassembly (#24125)

* Assembly: Prevent crash when link to linkgroup in subassembly

* ViewProviderAssembly: Additional crash failsafe.

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -997,6 +997,11 @@ AssemblyObject::getConnectedParts(App::DocumentObject* part,
 
         App::DocumentObject* obj1 = getMovingPartFromRef(this, joint, "Reference1");
         App::DocumentObject* obj2 = getMovingPartFromRef(this, joint, "Reference2");
+
+        if (!obj1 || !obj2) {
+            continue;
+        }
+
         if (obj1 == part) {
             auto* ref =
                 dynamic_cast<App::PropertyXLinkSub*>(joint->getPropertyByName("Reference2"));

src/Mod/Assembly/Gui/ViewProviderAssembly.cpp

@@ -794,6 +794,9 @@ ViewProviderAssembly::DragMode ViewProviderAssembly::findDragMode()
 {
     auto addPartsToMove = [&](const std::vector<Assembly::ObjRef>& refs) {
         for (auto& partRef : refs) {
+            if (!partRef.obj) {
+                continue;
+            }
             auto* pPlc =
                 dynamic_cast<App::PropertyPlacement*>(partRef.obj->getPropertyByName("Placement"));
             if (pPlc) {