I am using DNN Community 7.2.1

When I try to add a page, or do anything that uses the dropdown box that should list the pages, I get an Undefined type of error:

This has happened on both localhost and a production site.  

The Admin/Page Management part works ok, so I can add a page at the top level and then go in and change it there by dragging the page where it should be.  But any operation that uses the Pages dropdown box fails.

Any ideas anyone?

Thanks for your help.