Steps to reproduce:

1. go to "linux by zabbix agent" template
2. open trigger named "Zabbix agent is not available (for {$AGENT.TIMEOUT})"
3. make dependence of trigger ICMP Ping: Unavailable by ICMP ping
4. Update trigger(save changes)

Result:
Error: Cannot update trigger
Trigger "Zabbix agent is not available (for {$AGENT.TIMEOUT})" cannot depend on the trigger "Unavailable by ICMP ping", because the template "ICMP Ping" is not linked to the host "

".
See screenshot zbx.jpg

Expected:
I was expected that dependence is must working like in windows by zabbix ageny and in freebsd by zabbix agent