The pop-up window form display wasn't started by a process, just by the internal 'Edit Object' operation.
In the popped-up form, there is a multi ref attribute, 'Add New' button on that ref attribute starts a proccess. When the process is finished, I cannot close or cancel the pop-up form & get the error message.
When you view the form 'Edit Object' not in pop-up window, start the 'Add New' process, then there is no such error message.
Cheers