When you are in "Verfügbarkeit anzeigen" and you click the "go back" button in the server it takes you all the way back to the e-mail window.

Steps:

1. Create event.
2. Click on the "Teilnehmer" tab.
3. Click on "Verfügbarkeit anzeigen".
4. Click the "go back" buttom of the server.

Actual result:
You land in your e-mail inbox without warning.

Expected results:
Either: A warning pops up: Are you sure you want to leave this page, the event will not be saved?
Or: Only closes the "Verfügbarkeit anzeigen" window and you are still in the "Create new event".