When editing an experience app, ArcGIS Experience Builder does not allow adding new widgets, and the error message "Cannot read properties of undefined (reading 'manifest')" is returned on the widgets pane.

Workaround

1. Go to the page editor for medium-sized screens.
2. Change the layout from Custom to Auto.
3. Save the experience app.
4. Refresh browser.

Doing this resets the layout for the medium-sized screens page, but allows the addition of new widgets again. If the error still shows up after following these steps, consider deleting the problematic page, and recreating the page again.

Steps to Reproduce