Advancement Background shows a s "Null"

When using a data pack to add custom advancements/tabs, the background(s) will show up as the broken "null" texture rather than the image that should be called in the json file. I have attached the new forced crash log as well as a screenshot of the aforementioned Advancement Background "null" texture. Also since the "Affected Version/s" drop down box won't allow me to add any other affected versions, it also affects 1.16.1 and 1.16.2 as well.