Fix crash when enabling Advanced Tuning

Disable closing app on last window close. This will close
the app on every widget spawned in QML that closes (MessageBox)

src/AutoPilotPlugins/APM/APMTuningComponentCopter.qml

@@ -64,8 +64,8 @@ SetupPage {
             ExclusiveGroup { id: returnAltRadioGroup }
 
             Component.onCompleted: {
-                // Advanced tuning is hacked out due to Qt crash with Qml Charts and a QGuiApplication
-                //showAdvanced = !ScreenTools.isMobile
+                // We use QtCharts only on Desktop platforms
+                showAdvanced = !ScreenTools.isMobile
 
                 // Qml Sliders have a strange behavior in which they first set Slider::value to some internal
                 // setting and then set Slider::value to the bound properties value. If you have an onValueChanged

src/AutoPilotPlugins/PX4/PX4TuningComponentCopter.qml

@@ -29,8 +29,8 @@ SetupPage {
             width: availableWidth
 
             Component.onCompleted: {
-                // Advanced tuning is hacked out due to Qt crash with Qml Charts and a QGuiApplication
-                //showAdvanced = !ScreenTools.isMobile
+                // We use QtCharts only on Desktop platforms
+                showAdvanced = !ScreenTools.isMobile
             }
 
             FactPanelController {

src/AutoPilotPlugins/PX4/PX4TuningComponentPlane.qml

@@ -29,8 +29,8 @@ SetupPage {
             width: availableWidth
 
             Component.onCompleted: {
-                // Advanced tuning is hacked out due to Qt crash with Qml Charts and a QGuiApplication
-                //showAdvanced = !ScreenTools.isMobile
+                // We use QtCharts only on Desktop platforms
+                showAdvanced = !ScreenTools.isMobile
             }
 
             FactPanelController {

src/QGCApplication.cc

@@ -158,7 +158,11 @@ static QObject* shapeFileHelperSingletonFactory(QQmlEngine*, QJSEngine*)
 }
 
 QGCApplication::QGCApplication(int &argc, char* argv[], bool unitTesting)
+  #if defined(__mobile__)
     : QGuiApplication       (argc, argv)
+  #else
+    : QApplication          (argc, argv)
+  #endif
     , _runningUnitTests     (unitTesting)
 {
     _app = this;
@@ -555,9 +559,6 @@ bool QGCApplication::_initForNormalAppBoot()
 
     QSettings settings;
 
-    // Exit main application when last window is closed
-    connect(this, &QGCApplication::lastWindowClosed, this, QGCApplication::quit);
-
     _qmlAppEngine = toolbox()->corePlugin()->createRootWindow(this);
 
     // Safe to show popup error messages now that main window is created

src/QGCApplication.h

@@ -35,7 +35,25 @@ class QGCSingleton;
 class QGCToolbox;
 class QGCFileDownload;
 
-class QGCApplication : public QGuiApplication
+/**
+ * @brief The main application and management class.
+ *
+ * This class is started by the main method and provides
+ * the central management unit of the groundstation application.
+ *
+ * Needs QApplication base to support QtCharts module. This way
+ * we avoid application crashing on 5.12 when using the module.
+ * We don't have QtWidgets on mobile, avoid using it.
+ *
+ * Note: `lastWindowClosed` will be sent by MessageBox popups and other
+ * dialogs, that are spawned in QML, when they are closed
+**/
+class QGCApplication :
+      #if defined(__mobile__)
+        public QGuiApplication
+      #else
+        public QApplication
+      #endif
 {
     Q_OBJECT