Relax Breath of Solution.

The callLater() method queues an operation to be performed for the next screen refresh, rather than in the current update. Without the callLater() method, you might try to access a property of a component that is not yet available. The callLater() method is most commonly used with the creationComplete event to ensure that a component has finished being created before Flex proceeds with a specified method call on that component.

All objects that inherit from the UIComponent class can open the callLater() method. It has the following signature:
callLater(method:Function, args:Array):void

The following example uses a call to the callLater() method to ensure that new data is added to a DataGrid before Flex tries to put focus on the new row. Without the callLater() method

<?xml version="1.0" encoding="utf-8"?>
<mx:Application name="DataGridColumn_sortDescending_test"
        xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.controls.dataGridClasses.DataGridColumn;
            import mx.events.DataGridEvent;

            private function doHeaderRelease(evt:DataGridEvent):void {
                var column:DataGridColumn = DataGridColumn(evt.currentTarget.columns[evt.columnIndex]);
                DataGrid(evt.currentTarget).callLater(onCallLater, [column]);
            }

            private function onCallLater(column:DataGridColumn):void {
                columnSortDescending.text = column.dataField + ".sortDescending: " + column.sortDescending;
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:Label id="columnSortDescending" />
    </mx:ApplicationControlBar>

    <mx:DataGrid id="dataGrid"
            rowCount="4"
            headerRelease="doHeaderRelease(event)">
        <mx:columns>
            <mx:DataGridColumn id="col1" dataField="label" />
            <mx:DataGridColumn id="col2" dataField="data" />
        </mx:columns>
        <mx:dataProvider>
            <mx:Array>
                <mx:Object data="one" label="User 1" />
                <mx:Object data="two" label="User 2" />
            </mx:Array>
        </mx:dataProvider>
    </mx:DataGrid>

</mx:Application>