When using a Nik plugin, for example, CF Pro 4, on a smart object in Photoshop 2021, the canvas will disappear soon after applying the plugin and making other adjustments. The only way to see the image is to close the file and re-open it. If no Nik plugin has been applied PS works as expected. I have the latest version of everything (PS, Nik, Studio drivers) as of today’s date. This has happened since the release of PS 2021.