ASPxGridView - How to get a value of ASPxTextBox placed in DataItemTemplate

Hi

Need some help with a asp gridview control this is the snippet from the gridview columns:

ASPx
<dx:GridViewDataTextColumn VisibleIndex="0"  Name="Periode" Width="150px" Caption="Periode">  
                <DataItemTemplate>  
                    <dx:ASPxTextBox ID="periode" runat="server" Width="100%" Value='<%#Bind("Periode") %>'>  
                    </dx:ASPxTextBox>  
                </DataItemTemplate>  
            </dx:GridViewDataTextColumn>  
            <dx:GridViewCommandColumn ButtonType="Button" VisibleIndex="1" Width="60px" Caption="">  
                <CustomButtons>  
                    <dx:GridViewCommandColumnCustomButton ID="cmdMakeBon" Text="Maak bon">  
                    </dx:GridViewCommandColumnCustomButton>  
                </CustomButtons>  
            </dx:GridViewCommandColumn>  

What I want is when the button is clicked check if the textbox 'periode' is empty if so show popupcontrol else perform calback and execute code with the row values.
I got this as javascript but i does not seem to work how can I get the value from the aspxtextbox?

JavaScript
<script type="text/javascript">  

    var rowVisibleIndex;  

    function grid_CustomButtonClick(s, e) {  
        if (e.buttonID != 'cmdMakeBon') return;  

        rowVisibleIndex = e.visibleIndex;  
        s.GetRowValues(e.visibleIndex, 'Periode',ShowPopup);  
    }  


    function ShowPopup(rowId) {  
        popupControl.Show();  
       // btnYes.Focus();  
    }  


</script>