It seems that a previous version of Umbrello could create XMI files where some model objects were missing. This is a workaround so that Umbrello at least doesn't crash on loading such a file:

validateObjType(): Pass in the UMLObject* by reference. Add a Uml::IDType arg.
When the UMLObject* is NULL then create a new object with an artificial name, "LOST_" followed by the id, and set its ID to the arg passed in.

This means that the user needs to manually rename the LOST_ items after loading a corrupted file. Sorry.