It is not crashing on its own -- you are telling it to crash. On Windows (well, on Vista and greater -- the details may be slightly different on XP), here is what happens:
1. you click the Edit MXM button
2. MXED is shown
At this point, SketchUp is going to be waiting for you to finish with MXED.
3. you bring SketchUp to the front again, and it is not responding
4. for some unknown reason, you try to close the Scene Manager
5. Windows shows a dialog asking whether you want to crash SketchUp or not
The correct answer to that dialog is that you want to '
wait for for the program to respond.' Why? Because it is waiting for you to finish doing something that
you told it you wanted to do. It has to wait for you to finish with MXED, because when you're done, then I have to update the new MXM information in the Scene Manager!
If it were in my control, of course I wouldn't let you click the Scene Manager window's Close button (it does work that way on OSX) while you were in the MXED-editing loop, but it isn't.