Steps to reproduce:
1) Create template T1 with discovery rule, item prototype and trigger prototype (TRP1).
2) Create template T2 and link template T1 to it.
3) Create template T3 with item and trigger TR3.
4) Navigate to TRP1 and try to add dependency - TR3 (trigger from template T3).

Result:
Error message:

Expected:
No errors and dependency created
OR error message that AT3 is not linked to T1
OR some other clear error message.