QML application wizards: Add code to bail out on load errors

When something goes wrong with deployment or there is a syntax
error in the QML files, the old code would enter the main
loop not showing any windows. Worse, the binary is then locked
on Windows and cannot be overwritten. Add a clause checking
whether there are any root objects and bail out on failure.

Task-number: QTBUG-60764
Change-Id: I3620a09ec4331dc04a5194dfd2ece2ff44bdc429
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

share/qtcreator/templates/wizards/projects/qtcanvas3dapplication/main.cpp

@@ -6,6 +6,8 @@ int main(int argc, char *argv[])
 
     QQmlApplicationEngine engine;
     engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
+    if (engine.rootObjects().isEmpty())
+        return -1;
 
     return app.exec();
 }

share/qtcreator/templates/wizards/projects/qtquickapplication/main.cpp

@@ -10,6 +10,8 @@ int main(int argc, char *argv[])
 
     QQmlApplicationEngine engine;
     engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
+    if (engine.rootObjects().isEmpty())
+        return -1;
 
     return app.exec();
 }

share/qtcreator/templates/wizards/projects/qtquickcontrols2application/main.cpp

@@ -11,6 +11,8 @@ int main(int argc, char *argv[])
 
     QQmlApplicationEngine engine;
     engine.load(QUrl(QLatin1String("qrc:/main.qml")));
+    if (engine.rootObjects().isEmpty())
+        return -1;
 
     return app.exec();
 }