Fix exception: using indefinite braille messages
This fixes an exception when using the indefinite braille messages
feature. For more information see comment on PR #6669
(https://github.com/nvaccess/nvda/issues/6669#issuecomment-304929030)
STR:
- Make sure the default profile does not have the setting to show messages indefinitely.
- Create an app-specific config profile and choose the new feature to show messages indefinitely.
- Go to the application and trigger a message, e.g. by pressing NVDA+t to report the title bar.
- Go to the desktop or switch out of the application.
