Fix regression in type cast migration.
With commit a5f92b67 a regression has been introduced which removes UMLClassifier instances containing associations from their parent folder while loading from xmi file. On saving the uml model into a xmi file those instances are excluded because they are not in the parents object list (class member UMLPackage::m_objects). After (re)loading such files some LOST_xxxx items in the tree view indicates this issue. CCBUG:368282