I've created a Magento 2 widget with an image chooser (among other things). When I use this widget in the CMS and add it straight to a page (Content > Pages > Edit Page > Insert Widget), the image chooser will show up correctly, and allow me to select or upload an image.
However, if I go into the Widgets section of the CMS (Content > Widgets > Add Widget), trying to use the same image chooser part of the Widget gives
Uncaught ReferenceError: MediabrowserUtility is not defined.
In the searching that I've done, it seems that it's due to TinyMCE not being loaded on this page, but I can only find code references to fix it in Magento 1.