Commented Unassigned: ModalPopupExtender error [27771]

I am getting the following error after updating to 15.1

Sys.WebForms.PageRequestManagerServerErrorException: Extender control 'GeneralModalPopupExtender' cannot extend 'ClientEditBtnModalPopupHidden'. Extender controls of type 'AjaxControlToolkit.ModalPopupExtender' cannot extend controls of type 'System.Web.UI.HtmlControls.HtmlInputHidden'

Any thoughts on this?
Comments: This should really be listed as a breaking change somewhere in the documentation. ab8140 is not the only person who used HiddenFields for their ModalPopupExtenders to avoid exposing a clickable button in the UI.