asp.net - AjaxControlToolkit:ModalPopupExtender not working at second attempt -

i have created user control contains imagebutton.

upon clicking imagebutton open ajaxcontroltoolkit:modalpopupextender.

when run vb.net application , click on imagebutton .modalbackground css loaded, when close modalpopupextender , again click on imagebutton, on second load .modalbackground css not working.

.modalbackground {        background-color: gray;     filter: alpha(opacity=70);           opacity: 0.7;      } 

i had kind of problem, caused storing reference popup control in session. solution recursively find popup control looking in parent controls:

private t findparentcontrol<t>( t control )     t : class {     if( control.parent t )     {         return control.parent t;     }     return control.parent != null ? findparentcontrol( control.parent ) : null; }