This is a click on the "back" button in the edit check popup. For it to error like this, it needs to be in a state where said back button is accessible... but there's no open context item or inspector. This might mean that there's a reachable state where we just seem to have an empty edit check stage with the toolbar still showing... or it might mean that there's a way to access the toolbar after we think we've cleaned up and moved on.

Looks like it started happening with last week's train-deploy of wmf.10, so it's probably a side-effect of the changes made for the edit check API / multicheck.