DocumentWindow.Close failes w/o error

DocumentWindow.Close() is called using a Command. With simple documents this works and the eventhandler DockSite_WindowClosing is called. With some documents with complex Content the Close results in an empty DocumentWindow and DockSite_WindowClosing is not called.

Maybe there is an problem of some kind in my document, i dont know, but this failure doesnt show any message in the Debug/Output WIndow nor any exception is thrown. Have you any clue how i could at least get some error or reason for this behaviour?

Thanks in advance,
Martin