28 August 2013 by Web Bureau
ok - i ran into this issue the other day when using the modal popup extender on a masterpage and also within a user control. Not exactly the recommended way to do it but it works ok. in terms of the above error "SCRIPT5022: Sys.ArgumentNullException: Value cannot be null" this appears to happen if you have either a "OkControlID" or "CancelControlID" have values of controls that do not exist. Was pretty easy to resolve but not just as easy to find. Hope this helps someone out there.