Fix SIGSEGV in Assembly solver during document restore

AssemblyObject::onChanged() was calling updateSolveStatus() when the
Group property changed during document restore. This triggered the
solver (solve -> validateNewPlacements) while child objects were still
being deserialized, causing a segfault in validateNewPlacements() due
to accessing uninitialized data.

Add isRestoring() and isPerformingTransaction() guards matching the
pattern used by GroupExtension::onChanged() and other FreeCAD modules.

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -139,7 +139,10 @@ App::DocumentObjectExecReturn* AssemblyObject::execute()
 
 void AssemblyObject::onChanged(const App::Property* prop)
 {
-    if (prop == &Group) {
+    if (prop == &Group
+        && !isRestoring()
+        && getDocument()
+        && !getDocument()->isPerformingTransaction()) {
         updateSolveStatus();
     }
     App::Part::onChanged(prop);