Bugfix for #973, feature added as in #865.

If use custom zip directory is checked but no directory is chosen the default directory will be used.

Clicking on select for a custom directory won't render Sprite Builder useless: Cannot dismiss project settings dialog.

Fixes: https://github.com/spritebuilder/SpriteBuilder/issues/973
Implements feature: https://github.com/spritebuilder/SpriteBuilder/issues/865