Try this: Go change your default dashboard to anything other than the Overview dashboard. The sidebar button for the Overview dashboard disappears and can not be brought back!
change the dashboard back to the default dashboard and it will appear again.
If you need to access the default dashboard with a separate dashboard selected as the default. Go to
Settings → Dashboards → click “open” next to Overview.
@petro I certainly know that. My point is that the GUI should work just as with any other dashboard with the same pop up providing the choice to show it or not…