Delete qml engine prior to toolbox

src/QGCApplication.cc

@@ -448,6 +448,9 @@ void QGCApplication::setLanguage()
 
 void QGCApplication::_shutdown()
 {
+    // Close out all Qml before we delete toolbox. This way we don't get all sorts of null reference complaints from Qml.
+    delete _qmlAppEngine;
+
     shutdownVideoStreaming();
     delete _toolbox;
 }