There are 2 unrelated issues here:
The null issue
The Database Generated option should only be used if you explicitly setup the table itself to do so, and assuming you're using the default, embedded, database server (hsqldb), the table is not setup that way, therefore you should not use that option, but rather the Model Generated option (making use of a Database/Sequence Number element in the model), or the User Input option.
The save issue
I'm guessing this was a result of some undo/redo, and/or package rename actions. It would be great if you could provide specific instructions on how to recreate this.
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