Msa overview render task is not cancelled when one turn off overview

1. Open any long alignment.
Expected state: the MSA editor is shown, there is an overview.
2. If the alignment is long enough, then you can turn off the overview before the render task completes. Do it: click the "Overview" button on the main toolbar.
Expected state: the overview widget hides, the render task cancels.
Current state: the overview widget hides, the render task still works. If you change the msa (insert a gap somewhere), a safe point triggers.