I had the same problem today, found a solution to fill blank values in the database.
Under xxxxxxxx.wp_gdbpvc_cwa table; look for empty values under cwa_widget_header_wrapper column, fill them with h2 or whatever suitable for your case, then try to check again, make sure you purge cache, varnish if you use any plug-in.
I think it is also possible to fix this issue in the backend with editing all Custom Widget Areas one-by-one selecting a proper header wrapper.
