Fix crash when editing feature after opening a form with relation reference widget

Also fixes a leak/performance issue - the attribute table model
for the relation reference widget is not parented to the widget,
so it is never cleaned up when the widget is deleted.

(forward port from f976e8b)

Author: nyalldawson

src/gui/editorwidgets/qgsrelationreferencewidget.cpp

@@ -543,7 +543,7 @@ void QgsRelationReferenceWidget::init()
       attributes << mReferencedLayer->fields().lookupField( attr );
 
     layerCache->setCacheSubsetOfAttributes( attributes );
-    mMasterModel = new QgsAttributeTableModel( layerCache );
+    mMasterModel = new QgsAttributeTableModel( layerCache, this );
     mMasterModel->setRequest( QgsFeatureRequest().setFlags( QgsFeatureRequest::NoGeometry ).setSubsetOfAttributes( requestedAttrs, mReferencedLayer->fields() ) );
     mFilterModel = new QgsAttributeTableFilterModel( mCanvas, mMasterModel, mMasterModel );
     mFeatureListModel = new QgsFeatureListModel( mFilterModel, this );