FlightDisplayView: check if we have valid VideoReceiver
FlightDisplayView: check if we have valid VideoReceiver

ethz-asl/qgc_asl,ethz-asl/qgc_asl,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,ethz-asl/qgc_asl,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol