Ajax toolkit error - Uncaught TypeError: Cannot read properties of null (reading 'parentNode') at Sys.Extended.UI.ModalPopupBehavior._createDomElements

 Uncaught TypeError: Cannot read properties of null (reading 'parentNode')

    at Sys.Extended.UI.ModalPopupBehavior._createDomElements

if you find above error, than you have added visible property in panel to false.

<asp:panel runat="server" ID="pnlAddEdit"  CssClass="modal-fade" style="height: 80vh;overflow-y: auto;" BorderStyle="Double" Font-Size="Small" Visible="false">

Delete visible = 'false' property