Im getting that error when try to call a __doPostBack on one of my pages, every page that i have in the project use __doPostBack function but in this particular page im gett
I was getting this because my script was inside an Iframe that was being "disposed" (it was a Telerik "window" that was closed). The code defining _dopostback() inside my iframe was gone, but the code calling _dopostback() inside my iframe was still hanging around. Love that JavaScript!