The title explains it mostly, when I hit Create Composite for any non-combining accent, Glyphs 3 crashes. Even just for one master. In Font View or Edit View, both crash.
In which glyph did you run the command?
dieresis, grave, acute… Oddly, it works fine in dotaccent.
Do you have a dieresiscomb glyph? For me (3099), it works if one is present, otherwise the command does nothing, but not crash.
Yes, I have all my combining accents present in the file.
So you run the command on the “grave”? It is supposed to be used with the “Agrave” and so on.
And is the “gravecomb” build from a component?
I run it on the non-combining accents, which I want to be composites (combining accent as component). Glyphs agrees with this order (shapes in comb, non-combining accents as composites of comb). But running the command crashes glyphs in accents other than dotaccent, where it works fine.
As I said, can you check the combining accents if they are made from components themselves?
I am very stupid. Thank you 
The comb accents were components of the non-comb accents. So I was trying to make composites from composites that referenced the initial glyph.
I’ll have a look at the crash.