Data / Gimbal Video: may lock up if video stream is not available

While testing the live video display ahead of a search & rescue competition (Japan Innovation Challenge), I found that if the video stream was not available, the live video screen locked up Mission Planner completely and the only way to recover is to restart Mission Planner. This only happened the 1st time that I tried to use the feature. The 2nd time it seemed OK.

How to reproduce in SITL:

- start AP Copter-4.7.0-dev (or perhaps any version)
- param set SCR_ENABLE 1
- param set MNT1_TYPE 8 (Siyi)
- param set CAM1_TYPE 3 (Mount)
- copy the [video-stream-information.lua](https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Scripting/applets/video-stream-information.lua) script to the scripts directory
- param set VID1_CAMMODEL = 8 (Siyi ZT6 IR)
- restart the simulator and from MP select, Gimbal Video, Mini, Video Stream, Video
- if "Video" is not available in the drop-down use the Ctrl-F, "Message Interval" to request the VIDEO_STREAM_INFORMATION message at 1hz

FYI @robertlong13

Start by reproducing the lockup through Mission Planner's Gimbal Video, Mini, Video Stream, and Video path using the listed SITL and video-stream-information.lua steps. Trace the video-unavailable case and verify that opening the live video screen no longer locks Mission Planner and can be retried without restarting it.

Written by the indexing model from the issue text.