I am having this same exact problem. I first made the font with BMFont, and it looked like shit. So I replaced it and it still looked pretty bad. I tried to replace it again, and all of a sudden, every font is now throwing that error. All my previous fonts work (I can't preview any of the fonts because of that error), but I can't get the new font to work at all (it gets put on the atlas, but no text appears when running it). The font name is just "Apex_21", which didn't have a problem at all the last 2 tries. I don't know what GlyphDesigner is (a BMFont alternative?), so I can't just restart that. I tried remaking the Font in BMFont, but it still doesn't work. Maybe some type of issue occurred when I made the font bold in BMFont? I don't know. But now the font maker is completely broken and won't work at all.
Attached is the font. I made it bold, size 21 when it broke.
EDIT:In OnPreviewGUI on UIFontInspector, changing
Texture2D tex = mFont.texture;
to
Texture2D tex = null;
if(mFont != null)
tex = mFont.texture;
Seems to have fixed the error and the fonts now work again, and I can bring in new fonts. I feel this is just covering up an issue, but it will work for me for now.
EDIT 2:
I don't know if this has anything to do with it, but if you look at the preview image I attached (TextStretch.png), that is how the preview looks for just that one. It doesn't look like that in the actual text or the atlas, so it isn't really a big deal. But since the image sits at the very bottom of atlas, it is previewing strangely. Just thought I might want to point that out.