Sharepoint list can't exit classic experience

I'm guessing this is a resolved issue by now for many who have posted to this thread, but since I found this thread when I experienced the same problem (years later) and searched the web for a solution, I thought I'd post the solution I finally discovered.  I hope this is useful to others!

SOLUTION: Add ?contents=1 to the end of the page URL in your browser address bar.  This will take you to the Web Part Page Maintenance page.  Are there any closed web parts on the page?  Delete those web parts.  Return to the page.  You should now be able to exit the classic experience.