I tried to import a previously exported template with some discovery rules containing trigger_prototypes and the import failed with:

Undefined index: expression [api/classes/CTriggerPrototype.php:1288]
Undefined index: expression [api/classes/CTriggerPrototype.php:1290]
Cannot implode expression "".

I manually removed the <trigger_prototypes> sections from the template and it was imported succesfully.

I then tried to Full Clone the existing Template OS Linux, renamed to Template OS Linux 2 but when I clicked the "Save" button, it also failed with the same exact error.

I tested it with the latest 2.0.0 stable, then tested using the frontend from 2.0.1rc1 and with the frontend from 2.1.0 (trunk), they all failed with the same error (but slightly different line number on the exception probably due to code changes between versions).

The reported error was generated by Zabbix frontend 2.0.0 stable.