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'

Result
Nothing is shown, screen hangs indefinitely

Note, that the same graph can be added to the screen and displayed there just fine:

Note, that the preview of the graph that contains a single item also works fine.

Tested on:
Zabbix master 5.4
Ubuntu 18.04
MySQL: 5.7.32
Firefox 82.0 and Chromium 86 (also tested with force cache reload)