Cancelling Altitude Angel signup leaves MP lingering

When performing an Altitude Angel signup if you get stuck on (https://github.com/ArduPilot/MissionPlanner/issues/1376) you have no choice but close/cancel it but then Mission Planner gets stuck forever and needs to be force-closed.

Start by reproducing the Altitude Angel signup flow described here and compare it with issue #1376, then follow the cancellation path in Mission Planner. Done means cancelling or closing the signup leaves Mission Planner responsive and allows it to exit normally without force-closing.

Written by the indexing model from the issue text.