Event dialog validation

Minor bug fixes for...

* https://github.com/KSHSK/WAVED/issues/303
* https://github.com/KSHSK/WAVED/issues/287
* https://github.com/KSHSK/WAVED/issues/288
Yep that should be done here. We're already clearing the error flag whenever we call `hasErrors` so there's no reason not to clear that other error flag here as well since we check the condition for setting `eventEditorTriggeringWidgetError` a few lines down.