Looking at CustomReportsMenu.GetSource it appears that it is not finding your data source.
I can not tell from your screen shot if it is asking for the correct name? (the one you defined) Double click on ‘x’ in the debugger and see if Source is ‘Liste’
Did you restart Suneido to ensure the plugins are updated?
Did you name your plugin “Plugin_Reporter”? (It must be exactly this name.)