Workaround.
MainForm -> Script:
Ext.override(Ext.grid.column.RowNumberer, {
defaultRenderer: function(value, metaData, record, rowIdx, colIdx, dataSource, view) {
var newDataSource = dataSource.isFeatureStore ? dataSource.store : dataSource;
return this.callParent([value, metaData, record, rowIdx, colIdx, newDataSource, view]);
}
});