Exceptions should not break a UI

[denis-anisimov]

If exception happens in some event handler (f.e. as a reaction on some client-side event) then UI becomes broken because of internal error.

It's possible to install ErrorHandler via VaadinSession.setErrorHandler and be notified about exceptions but it's not possible to change UI since it becomes broken when an exception is thrown.

UI should remain functional.

[mstahv]

I find this "new behaviour" really nasty for both application developers and end users.

Even if the framework does nothing by default and just kills the UI, we should definitely have a hook for developers to catch "component errors" and let them handle and decide how that app behaves in such situations.

[samie]

For similar problematic error handling situation see also #4377

[CookedApps]

The UI still freezes in Vaadin 12.0.3 if an exception is thrown inside a ValueChangeListener. Example:

textField.addValueChangeListener(e -> {
    throw new RuntimeException();
});

I tested this with Flow version 1.2.3 and 1.3.0.alpha1.