Style is undefined in _getAttribution

[DoHe]

From our error logging I can see that relatively regularly style is undefined here.
An example:

TypeError: a is null
at _getAttribution/<(/map/js/chunk-vendors.ce2e5744.js:66:9590)

Which refers to the (sadly minified) line:

var s=[];for(var u in a.sources){var c=a.sources[u].url.split("mapbox://")[1];

I don't know yet how to reproduce but from our logging it seems to be 100% Firefox on Windows. Nearly all of if v67.
I didn't find any other mentions in the issues on this repo.