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while the FormMode is in Edit mode you cannot call on the Update method for the list. The code works fine in display mode on the DispForm.aspx page and it will hide and show the correct fields in the EditForm.aspx page but when you click Save that is when the error is thrown. If the Update method is removed the error will not be thrown however you will not be able to see the changes to the fields.

Check list.update() method and if it is unnecessary then remove it.