Fixes #2267. Toplevel.EnsureVisibleBounds throws an exception if border is null.

[BDisp]

Fixes #2267 - The check was only done on the if condition and not in the set.

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[BDisp]

@tig I updated the #2267 (comment) to show a visible issue that this PR will fix. Can you merge this to avoid making a cherry.pick on every branches where I'm working on, please. Thanks.