Fix null _activeVehicle references

src/FlightDisplay/FlightDisplayViewWidgets.qml

@@ -445,25 +445,25 @@ Item {
                 spacing: _margins
 
                 QGCButton {
-                    text:       _activeVehicle.armed ? (_activeVehicle.flying ? "Emergency Stop" : "Disarm") : "Arm"
+                    text:       (_activeVehicle && _activeVehicle.armed) ? (_activeVehicle.flying ? "Emergency Stop" : "Disarm") : "Arm"
                     onClicked:  _guidedModeBar.confirmAction(_activeVehicle.armed ? (_activeVehicle.flying ? _guidedModeBar.confirmEmergencyStop : _guidedModeBar.confirmDisarm) : _guidedModeBar.confirmArm)
                 }
 
                 QGCButton {
                     text:       "RTL"
-                    visible:    _activeVehicle.guidedModeSupported && _activeVehicle.flying
+                    visible:    _activeVehicle && _activeVehicle.guidedModeSupported && _activeVehicle.flying
                     onClicked:  _guidedModeBar.confirmAction(_guidedModeBar.confirmHome)
                 }
 
                 QGCButton {
-                    text:        _activeVehicle.flying ? "Land" : "Takeoff"
-                    visible:    _activeVehicle.guidedModeSupported && _activeVehicle.armed
+                    text:       (_activeVehicle && _activeVehicle.flying) ? "Land" : "Takeoff"
+                    visible:    _activeVehicle && _activeVehicle.guidedModeSupported && _activeVehicle.armed
                     onClicked:  _guidedModeBar.confirmAction(_activeVehicle.flying ? _guidedModeBar.confirmLand : _guidedModeBar.confirmTakeoff)
                 }
 
                 QGCButton {
                     text:       "Pause"
-                    visible:    _activeVehicle.pauseVehicleSupported && _activeVehicle.flying
+                    visible:    _activeVehicle && _activeVehicle.pauseVehicleSupported && _activeVehicle.flying
                     onClicked:  {
                         guidedModeHideTimer.restart()
                         _activeVehicle.pauseVehicle()
@@ -472,13 +472,14 @@ Item {
 
                 QGCButton {
                     text:       "Change Altitude"
-                    visible:    _activeVehicle.guidedModeSupported && _activeVehicle.armed
+                    visible:    _activeVehicle && _activeVehicle.guidedModeSupported && _activeVehicle.armed
                     onClicked:  _guidedModeBar.confirmAction(_guidedModeBar.confirmChangeAlt)
                 }
             } // Row
 
             QGCLabel {
                 anchors.horizontalCenter: parent.horizontalCenter
+                color:      qgcPal.button
                 text:       "Click in map to move vehicle"
                 visible:    _activeVehicle && _activeVehicle.guidedMode && _activeVehicle.flying
             }