Fix crash when using dialog with keyboard:

Fix crash when using dialog with keyboard:
The problem also happen with other dialogs. For example, open the document
dialog, press alt+p (pagestyle), and press enter -> The pagestyle button
becomes empty. Furthermore, when pressing "Apply", lyx will crash.
I havn't fixed the other dialogs, because I don't know if my fix is good
(maybe there is a simple fix to the problem).

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@2788 a592a061-630c-0410-9148-cb99ea01b6c8