Error: Sys.ArgumentNullException: Value cannot be null.
Parameter name: element
Then the fix is probably pretty simple - you have put incorrect ID's in either the OkControlID or CancelControlID attributes.
You don't have to specify the OkControlID or CancelControlID's to use the ModalPopupExtender but if you do and you later rename the control or you simply mistype when setting it up the compiler will not catch this.
The error message could be more helpful but hopefully you will have found this article in your favourite search engine without much trouble!
Here is the official documentation and demo page for this control: