This was a strange error which was daunting me for couple of hours today. The field name reported by Forms Builder had no formula attached to it.
The property of all fields in the block was also same. But since this was a warning, I tend to ignore. But after some more time, bogged over with the unnecessary popup while creating FMX.
Some googling gave me an answer that this was a Form Builder Bug. Then in no time, I got the solution from Oracle Forums.
Specify some dummy formula in the field which is reporting error, and later clearing the property to None and no Formula removed the warning thrown.
Recording for my reference.