Assets Catalog is Blank

Using xCode 13.4.1, my assets catalog (assets.xcassets) has gone blank. Which is to say when I click on the icon in the left-hand Navigator, the normal OutlineView and DetailView are replaced by a large white area of screen. If I try, for example, to drag a new image file into this area, it bounces out. The existing assets are still there; when I run the app the images, etc., appear as expected. I just can't see them to manage them (or add new assets). I have restarted xCode several times to no avail. Has this ever happened before? Does anyone have a solution? BTW, in other app projects, the assets catalogs are working normally. It is just in one project where the catalog is "blank"