Fix segfault on Linux at close for #3164 (#3166)

* Remove leftover debugging message
* Fix segfault on Linux at close for #3164

src/FirmwarePlugin/PX4/PX4FirmwarePlugin.cc

@@ -198,7 +198,6 @@ int PX4FirmwarePlugin::manualControlReservedButtonCount(void)
 
 bool PX4FirmwarePlugin::isCapable(FirmwareCapabilities capabilities)
 {
-    qDebug() << (capabilities & (MavCmdPreflightStorageCapability | SetFlightModeCapability | PauseVehicleCapability)) << capabilities;
     return (capabilities & (MavCmdPreflightStorageCapability | SetFlightModeCapability | PauseVehicleCapability)) == capabilities;
 }
 

src/ui/MainWindow.cc

@@ -282,6 +282,10 @@ MainWindow::MainWindow()
 
 MainWindow::~MainWindow()
 {
+    // This needs to happen before we get into the QWidget dtor
+    // otherwise  the QML engine reads freed data and tries to
+    // destroy MainWindow a second time.
+    delete _mainQmlWidgetHolder;
     _instance = NULL;
 }