Fix another 'Crash while importing C++ code from existing project'.

Exclude non UMLPackage based uml objects from accessing not present
m_objects member in Model_Utils::findUMLObject().

A common solution would be to solve the dynamic_cast<UMLPackage*>()
failure (see bug) or to add a virtual bool canHaveObjects() method
to UMLObject, which returns false by default and true for classes
derived from UMLPackage to guard the access to m_objects.

BUG:341709