QmlDesigner: crash fix

Task-number: QTCREATORBUG-8628
Change-Id: Id1e0984794b9ce590cfd5789c1cde6de7dc7bd1f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/qmldesigner/qmldesignerplugin.cpp

@@ -347,6 +347,9 @@ static bool isDesignerMode(Core::IMode *mode)
 
 void QmlDesignerPlugin::onCurrentModeChanged(Core::IMode *newMode, Core::IMode *oldMode)
 {
+    if (!Core::EditorManager::currentEditor())
+        return;
+
     if (Core::EditorManager::currentEditor()
             && Core::EditorManager::currentEditor()->id() != QmlJSEditor::Constants::C_QMLJSEDITOR_ID)
         return;