do not init dualview if no layer

fix crash of QtDesigner with custom widgets
kudos to @m-kuhn

src/gui/attributetable/qgsdualview.cpp

@@ -69,6 +69,9 @@ QgsDualView::QgsDualView( QWidget* parent )
 
 void QgsDualView::init( QgsVectorLayer* layer, QgsMapCanvas* mapCanvas, const QgsFeatureRequest &request, const QgsAttributeEditorContext &context )
 {
+  if ( !layer )
+    return;
+
   mEditorContext = context;
 
   connect( mTableView, SIGNAL( willShowContextMenu( QMenu*, QModelIndex ) ), this, SLOT( viewWillShowContextMenu( QMenu*, QModelIndex ) ) );