Hi there. Just started using grafana with prometheus and trying to make an alert for high disk space usage. I have a panel like this from node exporter full dashboard:
So i click “create new alert rule” for it and there all the fun starts 
When i do not change anything in query:
and click preview it shows me a timeseries just fine:
The problem is when i try to change legend or literally anything in a query, it shows an error:
But i just want to use {{job}} for a custom legend. Error occurs whenever I touch anything in the query. Even if I just switch from timeseries to table. Even I change it to previous state. It show a chart only on first click preview. Please, help me 