Unable to add OpenNMS Flows and Entities Datasources to Grafana: Cannot read property 'responseType' of undefined

Problem:
When attempting to add either the Flows or Entities Datasource into Grafana, the interface returns the error: Cannot read property ‘responseType’ of undefined

Expected outcome:
Successful adding of data source (which happens with the Performance Datasource, just not the Flows or Entities)

OpenNMS version:
v26
Helm - 5.0.1
Grafana - 6.7.0

I’ve trawled through the docs to try and find information regarding the setting up of these Datasources but it only describes adding them in, not if anything else needs to be enabled or turned on in ONMS for them to work. The error sounds like something is not configured but I can’t find any reference to what it might be.

To retirate, the Performance Datasource works fine, I get data from that into dashboards. The Flows and Entities DS both fail with the same error (as above). The same user/pass is used in all of them to auth with Grafana.