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"));