Get stuck when change class diagram when reopen saved concern with marks

[Tingyu Shen]

When re-open a concern with marks, user will get runtime error when try to change class diagram.

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Not sure if this is related to there is no stuff inside the classdiagram.editor and classdiagram.test packages on comparing-model branch

[Yong Zhang]

There is another problem that is similar. After multiple opens and closes of mark mode, sometimes we cannot create a new class. The options of creating do not pop up. Below is a picture. Might be caused by the same bug though.

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