Fix value() method of relation reference widget

Author: m-kuhn

src/gui/editorwidgets/qgsrelationreferencewidget.cpp

@@ -357,18 +357,7 @@ QVariant QgsRelationReferenceWidget::foreignKey() const
   }
   else
   {
-    if ( mReferencingFieldIdx < 0 || mReferencingFieldIdx >= mReferencingLayer->fields().count() )
-    {
-      return QVariant();
-    }
-    else if ( !mFeature.isValid() )
-    {
-      return QVariant( mReferencingLayer->fields().at( mReferencingFieldIdx ).type() );
-    }
-    else
-    {
-      return mFeature.attribute( mReferencedFieldIdx );
-    }
+    return mComboBox->identifierValue();
   }
 }