[MWF] Unit test for DataGridView bug (#19567)
[MWF] Clear currentCell before disposing (fixes #19567)

If currentCell still has a value when we call Columns.Clear() we
might try to end the editing. Because the columns got disposed
before calling Columns.Clear() we get a crash. This patch fixes
the problem by setting currentCell to null.
Merge pull request #1018 from ermshiperete/xamarin-19567

[MWF] Clear currentCell before disposing (fixes #19567)