Steps to reproduce:

1. Create discovery rule with 2 checks:
2. 1 check is SNMP
3. 2 check is other, for example Telnet.
4. Select radios:
   1. Device uniqueness criteria => SNMP,
   2. Host name => SNMP,
   3. Visible name => SNMP,
5. Save the form.
6. Reopen saved Network discovery.
7. Change Telnet port to some new value.
8. Try to save the form.

Result:
Error message

Incorrect value for field "name_source":