It might be better to return { width: 0, height: 0, center: [0, 0], low: [0, 0], high: [0, 0] } instead since according to the types, center, low, and high aren't nullable

Thanks for the PR!
But, since the whole library doesn't have 100% test coverage (my fault not yours), I'm a little hesitant to take this. Perhaps you could add your own defensive function in your own code:

function alwaysMeasureModel(model) {
   return makerjs.measure.modelExtents(model) || { width: 0, height: 0, center: [0, 0], low: [0, 0], high: [0, 0] };
}

Or perhaps the return type of modelExtents should be changed to IMeasureWithCenter | null? I created this PR mostly because I got bitten by this in production

Sorry it bit you in production. Yes the | null is a good approach.

Updated PR. Also updated method to explicitly return null