Fixes #1999. Prevents the mouseGrabView being executed with a null view.

[BDisp]

Fixes #1999 - There were two bugs. Firstly on ScrollView which was returns false after the UngrabMouse even if the MouseEvent.View property isn't null and thus didn't run the view's mouse event. Secondly the mouseGrabView was run with the MouseEvent.View property equal to null. Now if the view is null the mouseGrabView will also be null by running the UngrabMouse method.

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[tig]

LGTM. Nice work.