spark's datagrid gets blank

hi guys ...

iam create application and add flex spark datagrid to it using flash builder 

when i run application it works fine and data showed in the datagrid but 

when i rotate it right or left datagrid becomes blank and rows empty
 

here's my simple syntax

<s:DataGrid id="DGrid" width="368" height="210">

        <s:dataProvider>

            <s:ArrayList>

                <fx:Object FirstName="esraa" LastName="Hashem"/>

                <fx:Object FirstName="eman" LastName="Mustafa"/>

                <fx:Object FirstName="hend" LastName="Ahmed"/>

                <fx:Object FirstName="samar" LastName="Naeem"/>

                <fx:Object FirstName="abeer" LastName="Hassan"/>

                <fx:Object FirstName="mona" LastName="Mohamed"/>

            </s:ArrayList>

        </s:dataProvider>

    </s:DataGrid>

so what's the problem & how can i fix that ?