FlighData HUD widget can be closed without chance to open again

#### Issue details
1. In Flight Data view swap HUD and Map with rightclick menu on HUD
2. Undock HUD with doubleclick on it
3. Close HUD with [X] button
--> You have no HUD on screen and have no control to show it.

Workaround - resize map area up by dragging bottom edge upward, then piece of HUD will appear and then Swap with map by rightclick menu.

#### Version
All versions

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

#### Airframe type
All

#### Hardware type
All

#### Logs
No log needed

Reproduce the sequence in the Flight Data view: swap the HUD and map, undock the HUD, then close it with the X button. Trace the HUD/map swap, undocking, and close behavior; done means the HUD can still be reopened without using the map-resize workaround.

Written by the indexing model from the issue text.