Steps to reproduce:

Not sure if this reproducable without my data, but this is what I do to reproduce it:

1. Edit template (which I created in v3.x, adapted to dependent items in 4.0)
2. Open triggers-tab
3. Edit trigger checking for nodata on master-item
4. Change master-item into one of the dependent items (as master-item has history set to 0 so nodata triggers always..)
5. Click on Apply

Result:

Empty webpage

error in Apache errorlogging:

PHP Fatal error: Uncaught TypeError: Argument 2 passed to dbConditionString() must be of the type array, null given, called in /usr/share/zabbix/include/classes/api/services/CTriggerGeneral.php on line 1293 and defined in /usr/share/zabbix/include/db.inc.php:911\nStack trace:\n#0 /usr/share/zabbix/include/classes/api/services/CTriggerGeneral.php(1293): dbConditionString('i.key_', NULL)\n#1 /usr/share/zabbix/include/classes/api/services/CTriggerGeneral.php(1012): CTriggerGeneral->implode_expressions(Array, Array, Array, true)\n#2 /usr/share/zabbix/include/classes/api/services/CTriggerGeneral.php(198): CTriggerGeneral->updateReal(Array, Array, true)\n#3 /usr/share/zabbix/include/classes/api/services/CTriggerGeneral.php(76): CTriggerGeneral->inheritOnHost(Array, Array)\n#4 /usr/share/zabbix/include/classes/api/services/CTrigger.php(589): CTriggerGeneral->inherit(Array)\n#5 /usr/share/zabbix/include/classes/api/clients/CLocalApiClient.php(123): CTrigger->update(Array)\n#6 /usr/share/zabbix/include/classes/api/wrappers/CFrontendApiWrapper in /usr/share/zabbix/include/db.inc.php on line 911, referer: http://zabbixtstfe.uantwerpen.be/triggers.php?form=update&triggerid=18463

Expected:
Trigger being updated in template and on all hosts that have that template applied to.