Stop disposing a dialog while showing it

This fixes FWNX-983 "My FW project always hangs whenever working with
'Find in Dictionary' function".  The program freezes up in Mono when a
dialog is disposed while it is in the ShowDialog method.  I still can't
believe this didn't cause problems on Windows!

Change-Id: Ic4a9ca9aa5b5539552d3fbca7cfbfc0026b43cbf