When editing iSCSI extent with Extent Type = File, changing dropdown value to "Device" yields in "null_value" option being added and selected to the "Device" field
I expect this field to be set empty (not "null_value"), and empty option not to be available for selection anymore once device is selected.
I suspect there is a similar behavior in other components. "null_value" is not a human-readable string and should not be exposed. Non-nullable field should not be allowed to have null once the selection was made.