ModalPopupExtender closes abruptly on RadioButtonList item multi-click

I've a ModalPopupExtender which has RadioButtonList in it with values True & False with AutoPostBack = 'true'. This is inside a UpdatePanel. On the OnSelectedIndexChanged event of their individual click it does a certain steps (It does work appropriately). But, when either of the values are clicked more than one time (multi-clicked), then this AJAX Modal Popup closes.

Please suggest a workaround.