Type Ba in the search box and flashback will appear！

This is a problem with the rendering of items in some mods. They require a world to be loaded, however in the main menu it is obviously not loaded. If you can determine the exact mod causing the issue, I can add an exception for it. The best way would be to slowly scroll down the list until the "flashback" appears. It's in alphabetical order, so should be easy to figure it out.