How to Restart Web Server After Google App Engine Launcher Crash

1. Open a terminal window.
2. Copy and paste this: sudo lsof -i -n -P | grep TCP
3. Take note of the process id (PID) currently listening on the port your app was running on (look for something that looks like: TCP 127.0.0.1:8080 (LISTEN)).
4. Open Activity Monitor.
5. Sort the processes list by PID.
6. Kill the process matching the PID you noted in step 3.
7. Restart your server from Google App Engine Launcher.

Thanks to irs8925 for providing the answer on Stack Overflow.