Merge pull request #6148 from MichalZr6/fix_remove_artifacts

Minor fixes for artifact removal during map load and handling null map objects

mapeditor/mapcontroller.cpp

@@ -746,6 +746,8 @@ ModCompatibilityInfo MapController::modAssessmentMap(const CMap & map)
 
 	for(auto obj : map.objects)
 	{
+		if(!obj)
+			continue;
 		modAssessmentObject(obj.get(), result);
 	}
 	return result;