QmlDesigner: Fix crash when switching to first .ui.qml file

if Qt Creator is already in Design mode.

QmlDesignerPlugin::showDesigner() was only called on a switch to Design
mode while the editor is a .ui.qml file. So it wasn't called when opening
a .ui file in Design mode and then switching to a .ui.qml file. But
showDesigner() creates the QmlDesigner widgets, so it must be called.

Fixes: QTCREATORBUG-32854
Change-Id: I02a498ce00fc1afdee900752ade7af6c62b787b7
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/designmodewidget.cpp

@@ -651,6 +651,11 @@ void DesignModeWidget::setMinimumSizeHintFromContentMinimumSize(bool value)
         dockContainer->layout()->update();
 }
 
+bool DesignModeWidget::isInitialized() const
+{
+    return m_initStatus == Initialized;
+}
+
 void DesignModeWidget::dragEnterEvent(QDragEnterEvent *event)
 {
     event->accept();

src/plugins/qmldesigner/designmodewidget.h

@@ -75,6 +75,8 @@ public:
 
     void setMinimumSizeHintFromContentMinimumSize(bool value);
 
+    bool isInitialized() const;
+
 signals:
     void navigationHistoryChanged();
     void initialized();

src/plugins/qmldesigner/qmldesignerplugin.cpp

@@ -529,8 +529,15 @@ void QmlDesignerPlugin::hideDesigner()
 
 void QmlDesignerPlugin::changeEditor()
 {
-    clearDesigner();
+    if (d->mainWidget.isInitialized()) {
-    setupDesigner();
+        // showDesigner was already already called
+        clearDesigner();
+        setupDesigner();
+    } else {
+        // we are already in Design mode, but showDesigner wasn't called yet,
+        // so we need to call that to set up the widgets
+        showDesigner();
+    }
 }
 
 void QmlDesignerPlugin::jumpTextCursorToSelectedModelNode()