I don’t know how much help I can be in diagnosing/providing reproduction steps for this bug, as I haven’t seen a consistent pattern in how/why it occurs, but it hit me maybe 3 or 4 times in the course of hinting a few hundred glyphs. so - not bad, all things considered.
What happens is, when doing TrueType hinting in the editor view, I may finish a glyph, and close the editor window, go back to the Font view, and double click another glyph to go back to hinting or editing or whatever — but the glyph won’t appear in the editor or the preview panel, both will be blank. Restarting Glyphs always solves the problem. Possibly related to having the TrueType tool selected while going back into the Font view? Just guessing.
More of a small nuisance than anything else, but figured I’d mention it in any case.