The following error message appears when attempting to execute the List method of an SQL table based SmartObject:
"Column '%Column_Name%' does not allow nulls"
The error only appears when the List method gets executed. Create, Delete, and Get record methods are working fine.
A SmartObject with exactly the same structure works correctly in another environment.
Based on the symptoms above, you may need to verify the records in the column mentioned in the error message. (Check for empty values or values in the wrong format) as one "bad record" is enough for the List method to fail.)
The fact that other methods are working indicates that there are no issues with the SmartObject itself, but rather a problem caused by data. You can resolve this by deleting the incorrect entry / entries.