Exception on tap-and-hold on validation error

[Matthias Schoettle]

It is possible to trigger an exception doing the following:

1. Create a model with a validation error
2. Undo so that the validation will disappear (after the delay)
3. Tap-and-hold on error to receive quick fixes

If the tap-and-hold is performed just while the validator validates and removes the error, an exception occurs, because while the event is processed, the target of the event is removed.

I suspect that the removal is not performed on the UI thread. It might be enough to perform that using invokeLater.