Deleting item used in trigger prototype results in corrupt expression

To reproduce:

• Create trigger prototype.
• In trigger expression use one item prototype and another normal item.
• Save trigger prototype.
• Now delete the normal item.

This results in corrupted trigger prototype expression like {host:item.last()}=0 and *ERROR*=0.

Deleting an item used in trigger expression also deletes trigger. It should also check if used in trigger prototypes and delete trigger prototype as well.