Fix for DataTable race-condition crash

[jamespan]

This patch fix some race condition crashes not fixed in #1962.

When updating a cell in DataTable with update_width=True, then remove the row, a crash will happen when on_idle run.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[rodrigogiraoserrao]

Hey there, thanks for opening this PR.

I added a small request after a quick look at your changes.
(By the way, thanks for adding a test!)

[willmcgugan]

@darrenburns Your call on this one...

[darrenburns]

I wonder if adding the following to remove_row is all that's needed here.

        for column_key in self._data.get(row_key):
            self._updated_cells.discard(CellKey(row_key, column_key))

This ensures that when we remove a row, then any cells that were contained within that row and awaiting a new width calculation are discarded before on_idle comes around.

[willmcgugan]

I think we are going to go with an alternative solution. Thanks for bringing this to our attention @jamespan