I reported this in the previous version of Bizagi Modeler and it seems to happen in the latest Bizagi modeler too. Occasionally I get this pop up with Object reference not set to an instance of an object which gives me an option to Continue or Quit, and usually I continue so I could save the work I did. However, the icons in the diagrams seems to freeze so I couldn't proceed working, I would have to Save, Close and Reopen the same file.
I have the screenshot and the JIT stack trace, unfortunately attaching that to this post will crash my IE8 (I wonder why). Happy to send via email to whoever is working on this case.
Hopes this issue gets resolved.