Redo the notification popup implementation
As there is the following warning: info: Don't use 'BuildContext's across async gaps. (use_build_context_synchronously at [mobileeventlog] lib/addevent.dart:303)
Additionally add a red error popup with the text "Configuration required" on the overview page which is displayed in case "Add Event" is clicked but the devices and event types are not downloaded/configured yet.
Make it reusable on the different Widgets.
Edited by Maximilian Betz