Panel plugin not found: pmm-singlestat-panel

I am trying to use the dashboards from https://github.com/percona/grafana-dashboards with Grafana (v6.3.0-beta1) by importing the json files, but on the MySQL Overview dashboard the Buffer Pool Size of Total RAM panel shows the error “Panel plugin not found: pmm-singlestat-panel” - how can I fix this?

Chris