QQuickLoader: Check for QQmlEngine before using it

The loader's context may have been removed from the context hierarchy or
it may not have a context in the first place. We should not crash then.

Pick-to: 5.15 6.2 6.3
Fixes: QTBUG-67950
Change-Id: I1058d5b1f978aa040f8b2f018c4357dd7a3ef333
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/quick/items/qquickloader.cpp

@@ -1041,9 +1041,15 @@ void QQuickLoaderPrivate::createComponent()
     const QQmlComponent::CompilationMode mode = asynchronous
             ? QQmlComponent::Asynchronous
             : QQmlComponent::PreferSynchronous;
-    QQmlContext *context = qmlContext(q);
-    component.setObject(new QQmlComponent(
-                            context->engine(), context->resolvedUrl(source), mode, q), q);
+    if (QQmlContext *context = qmlContext(q)) {
+        if (QQmlEngine *engine = context->engine()) {
+            component.setObject(new QQmlComponent(
+                                    engine, context->resolvedUrl(source), mode, q), q);
+            return;
+        }
+    }
+
+    qmlWarning(q) << "createComponent: Cannot find a QML engine.";
 }
 
 #include <moc_qquickloader_p.cpp>

tests/auto/quick/qquickloader/data/noEngine.qml

@@ -0,0 +1,32 @@
+import QtQuick 2
+
+Item {
+    id: root
+    property bool a: false
+    property int changes: 0
+    onAChanged: {
+        m.model = 0
+        m.model = 1
+        ++changes;
+    }
+
+    Repeater {
+        id: m
+        model: 1
+
+        Item {
+            Timer {
+                onTriggered: {
+                    root.a = true
+                    l.source = "loaded.qml"
+                }
+                interval: 0
+                running: true
+            }
+
+            Loader {
+                id: l
+            }
+        }
+    }
+}

tests/auto/quick/qquickloader/tst_qquickloader.cpp

@@ -134,6 +134,7 @@ private slots:
 
     void setSourceAndCheckStatus();
     void asyncLoaderRace();
+    void noEngine();
 };
 
 Q_DECLARE_METATYPE(QList<QQmlError>)
@@ -1533,6 +1534,20 @@ void tst_QQuickLoader::asyncLoaderRace()
     QCOMPARE(loader->item(), nullptr);
 }
 
+void tst_QQuickLoader::noEngine()
+{
+    QQmlEngine engine;
+    const QUrl url = testFileUrl("noEngine.qml");
+    QQmlComponent component(&engine, url);
+    QVERIFY2(component.isReady(), qPrintable(component.errorString()));
+    QScopedPointer<QObject> o(component.create());
+
+    const QString message = url.toString()
+            + QStringLiteral(":27:13: QML Loader: createComponent: Cannot find a QML engine.");
+    QTest::ignoreMessage(QtWarningMsg, qPrintable(message));
+    QTRY_COMPARE(o->property("changes").toInt(), 1);
+}
+
 QTEST_MAIN(tst_QQuickLoader)
 
 #include "tst_qquickloader.moc"