I am getting this error whwnever I try to select system.memory.actual.used.pct from metricbeat index.
I met the same issue.
Currently looking into this issue, it appears to happen only when you type in the search input box to limit results and then choose a selection. If you choose a selection without searching it will work for now or you can still search but use your arrow keys and Enter to select instead of clicking with mouse.
Update: It appears to be related to the component library we are using. We are tracking the issue here: