Wizards: Handle errors more elegantly in Qt Quick application template

By monitoring the objectCreated() signal of QQmlApplicationEngine we can
also catch errors if the components are instantiated asynchronously.

Task-number: QTBUG-39469
Change-Id: I796eb5561fc8d48ab5aa74d37b0964b118a1ba7e
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -20,9 +20,13 @@ int main(int argc, char *argv[])
     QGuiApplication app(argc, argv);
 
     QQmlApplicationEngine engine;
-    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
-    if (engine.rootObjects().isEmpty())
-        return -1;
+    const QUrl url(QStringLiteral("qrc:/main.qml"));
+    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
+                     &app, [url](QObject *obj, const QUrl &objUrl) {
+        if (!obj && url == objUrl)
+            QCoreApplication::exit(-1);
+    }, Qt::QueuedConnection);
+    engine.load(url);
 
     return app.exec();
 }