Assembly: Fix "deactivated by activating a App::Part, assembly stay in edit"

Fix "Activating a body in a part in an assembly deactivates the assembly and activate the part"
Fix "A manually deactivated assembly is still restoring later"

src/Mod/PartDesign/Gui/Utils.cpp

@@ -284,7 +284,11 @@ App::Part* getActivePart()
 {
     Gui::MDIView* activeView = Gui::Application::Instance->activeView();
     if (activeView) {
-        return activeView->getActiveObject<App::Part*>(PARTKEY);
+        auto* obj = activeView->getActiveObject<App::Part*>(PARTKEY);
+        if (!obj) {
+            obj = activeView->getActiveObject<App::Part*>(ASSEMBLYKEY);
+        }
+        return obj;
     }
     else {
         return nullptr;