Toggle Safety Switch doesn't work recently

#### Issue details
"Toggle Safety Switch" button on Testing Screen doesn't work for recent versions of MP.
It works fine on mavproxy `arm safetyoff`
#### Version
1.3.66 - 1.3.67

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

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

Reproduce the failure from the Mission Planner Testing Screen on Plane with Pixhawk hardware, using the affected 1.3.66–1.3.67 versions if available. Compare the button's behavior with the working mavproxy `arm safetyoff` command, then verify that the Toggle Safety Switch action works and does not regress other vehicle types.

Written by the indexing model from the issue text.