This font does not work on some systems ("… is not a valid font") due to invalid characters ("noncharacters"). To fix this, you can use this tool: https://fontfix.sslab1.workers.dev/ . This can also be used for other broken fontstructions as well.

Thanks, but your method doesn't seem to work. I believe the characters are PERMANENTLY in my font. I've created a new font, added the noncharacters, and tried your method on it, and when downloading it still says the font is invalid. The new font also says it has “13 glyphs”, meanwhile it only has 11 (10 glyphs + 1 space I modified and deleted). Maybe this used to work, but doesn't now?

Here is the networks tab of my fontstruct. I literally just found this. The codepoints U+FFFE (65534) and U+FFFF (65535) are sent, as well as space (32) and other blank characters.

Thank you so much! Why is the “Remove empty glyphs” hidden in a secret place that I have never touched?? It seems to have fixed the broken font now. 
Also after testing it seems like certain glyphs are still permanently there (whitespaces).