MP/Altitude Angel crash on network disconnect.

#### Issue details
Crash on loss of net connection. Booting MP with a net connection and altitude angel enabled, and then changing network to one without an internet connection (such as when connecting to a wifi rtk base station) results in MP crash.

#### Version
1.3.5

#### Platform
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_

#### Logs
_Please provide a link to any relevant logs that show the issue_

No source files, tests, entry points, logs, or stack trace are identified. First reproduce the reported sequence in Mission Planner 1.3.5 with Altitude Angel enabled, switch from an internet-connected network to one without internet access, and capture the crash details; done means the transition no longer crashes Mission Planner.

Written by the indexing model from the issue text.