Fixed bug with entering or double clicking an empty node selection in the selection dialog.

This would cause an IndexArrayOutOfBoundSelection before. Now, the dialog acts as if nothing happened and awaits further input.
Changed: The Quit button's behavior in the Exception dialog was a bit tricky.

Previously it would exit the application entirely without warning the user. Now it asks whether changes should be saved. (which is the default behavior of Quit in the application menu anyway).