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