Steps to reproduce
- Go to Configuration->Hosts->Graphs->Create Graph
- Give it a Name
- Add all Items that exists by default
- Click Add
- Go back to the same Graph and click 'Preview'
Note, that the preview of the graph that contains a single item also works fine.
Zabbix master 5.4
Firefox 82.0 and Chromium 86 (also tested with force cache reload)