When starting the latest snapshot of Horizon 30 via Docker (i.e., opennms/horizon:bleeding, I get "null" on the graphs page.
To reproduce the problem, start a simple lab via Docker, for instance:
As you can see, I'm using the defaults (no changes in OpenNMS configuration besides access to the database).
By default, OpenNMS will add a node to monitor itself. You can add more nodes if you need them. From the node page, when you click on Resource Graphs, then either select a few and click Graph Selection or use Graph All, the URL you'll see on the Browser's navigation bar has "null" instead of "graph/results.htm" which is what OpenNMS must use.
If you manually update the URL, I can see the graphs appropriately rendered.
I'm having issues compiling OpenNMS from source, so I don't know if the problem is associated with the Docker deployment or not.
I even tried using a Private Browser Tab to discard potential cache-related problems, and the problem persists.