Dropdown validation error

When I tried to update a record I got the following message:
Dropdown validation error: attempt to insert a value that is not one of the available options.

This has to do with the field type select_single_radio.
Normally when you make a new item, you choose one of the option. But when you omit to do so, then you get this error.
The error doesn't show what field causes the error.
Solution: make this type of field always a required field.
 
Correction
It has to do with the field type select_single(_radio) and with Hard-coded list of the options:.
If the option ~~ (blanc) is not included, you get the error.
 
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