Bugfix
If I drag an object type from another module into the current module and drag a reference from the "foreign" object type to an object type living in the current module, the editor displays a null pointer exception.
However, the relationship is still created and is displayed in the diagram of the other module.
Test
Create two modules Module1 and Module2 with one type each. Then drag and drop the type of the Module2 module into the diagram of the Module1 module (via the type tree of the model editor). Both can then be seen in the diagram. Create a reference from Module2 to Module1. The reference should be created without an error message.