Go to https://developers.facebook.com/
Enter Your facebook username and password
Go to My Apps dropdown menu to find Developer settings
In Contact Tab page Scroll bottom to see Delete Developer Account Panel
Click Delete Account Button
Finally click Save Changes button
Note:If you're the only admin of an app, the app will be deleted as well.
I just tested this myself, using your event.
I also got the error message - but when I scrolled down there was also a Get Code button (as you did) - and I was able to use it to get code.
Perhaps you should try getting the code and installing it.
The other thought I would have is that you are using the Embed Post option to try to embed an Event. But a ...
From a technological perspective, the important problem which you are addressing -- that of a disgruntled admin "hijacking" a Facebook application -- does not seem to currently have a solution.
This problem is virtually identical to that of a Facebook page being similarly hijacked, and as mentioned in this article on Mashable from Sept. 2011, the latter ...
Deleting oneself (the only administrator) from one of the test applications results in
Must specify at least one developer who has administrator permission.
So it's possible that the original administrator deleted their Facebook account or it got suspended. Or your app was victim of this bug: http://bugs.developers.facebook.net/show_bug.cgi?id=18848