Extensions that create a Widget subclass (using the AdminWidgetHook) can throw exceptions inside Reviewboard. The parts of Reviewboard that call those methods have been sandboxed.
Now when a Widget subclass from an extension throws an exception; Reviewboard logs the errors with enough information to find which method in the Widget subclass threw the exception.

Unit tests have been written to make sure that functions from a Widget subclass have been called, and when an exception is thrown it gets logged.
The tests fail without the sanboxing, and succeed with it.
Admin UI displays the widgets in the correct column.

When the "X" in the corner is clicked it closes the widget, but does refresh the page.