Steps to reproduce:

1. Add new network discovery
2. Add "FTP" check with default port range (or other)
3. Add "Zabbix agent" check with agent.ping key
4. Change "Host name" and/or "Visible name" option to check "Zabbix agent "agent.ping""
5. Save discovery
6. Edit discovery
7. Edit FTP check and update without changes (or change check)
8. Try to save discovery

Result:
Cannot update discovery rule. Incorrect value for field "name_source": _8.

Expected:
Discovery rule updated