Throw better exceptions when adding null LegacyBarActions

[Cheesebaron]

Spent some time finding out why I got a null pointer exception in LegacyBar, and found out it was because of I was trying to add an Action to the LegacyBar which was null. Had we had a better exception in hand here, it would have been a bit easier to understand why the exception was thrown.

Maybe add an ArgumentException in the methods for adding Actions to the Bar.