In fields with value map widgets, NULL is accepted as valid even if the constraint "not null" is enabled. Works as expected on QGIS Desktop.

[gioman]

Description

Take a field with a value map. The value map has also an entry for the NULL value (added by clicking "add NULL value")

then also enable the "not null" constraint

In QGIS Desktop all works as expected, if a value from the list is first selected (manually or using a default value), then the user chooses the NULL entry, the constraint does is job:

On MM App the NULL entry from the list is accepted as a valid value:

If the "NULL" entry from the value map is the default value then the constraint works also in MM App (but not if the user changes to another entry then goes back to "NULL").

Not even evaluating "NULL" as text in a custom expression (rather than using the normal "not null" constraint) helps workaround this problem.

Application (+ app version, build, operating system)

2026.2.0