Steps to reproduce:
1) Create item with type - Zabbix agent and key - agent.hostname (select from the list)

Result 1:
Check that "Trend storage period" option disappeared.

2) Press Add to create item.
3) Open created item again and check that "Trend storage period" appeared again.
4) Change "Trend storage period" to "Storage period" and press Update.
5) Open item again.

Result 2:
Field appears.

Expected:
Trend field doesn't appear at all.