Null Error when uploading my mod files

When I upload my mod file to the "Upload Mod" section, it works fine. But when I say to share it publicly, it says I have an error.

I check what it is and it says that the mod files should not be null. But, I put my mod in there before and it seemed to have removed it. I put it back in, say upload mod again and it deleted my mod file I put in and says it is null. How do I bypass this? I am putting my mod file in!!!