Correctly set parent for select by expression dialog in main window

(cherry-picked from ac2cd47)

src/app/qgisapp.cpp

@@ -6167,7 +6167,7 @@ void QgisApp::selectByExpression()
     return;
   }
 
-  QgsExpressionSelectionDialog* dlg = new QgsExpressionSelectionDialog( vlayer );
+  QgsExpressionSelectionDialog* dlg = new QgsExpressionSelectionDialog( vlayer, QString(), this );
   dlg->setAttribute( Qt::WA_DeleteOnClose );
   dlg->show();
 }