…ewer (#15214)

Fixes regression from #14503

Summary of the issue:
As part of #14503, braille extension points have been added that are used by the braille viewer. However when destroying the braille viewer by pressing alt+f4, the display size filter handler was never unregistered, resulting in the braille handler still assuming 40 braille cells even though no display was connected.

Description of user facing changes
Braille handler connectly restores to 0 cells again after disabling the braille viewer with alt+f4 when no braille display is connected.

Description of development approach
Changed if check on the dialog to is not None explicitly. wx gives a destroyed window a bool value of False, so the boolean check would work.
Ensure braille.filter_displaySize is unregistered when destroyBrailleViewer is called when the window is already destroyed. Note that destroying the window will unregister braille.pre_writeCells properly, so that unregistration can be limited to an undestroyed window only.