Hi:

I recently notice when I use ts-mixer (https://www.npmjs.com/package/ts-mixer) to construct the Model with a trait, eg:

class ModelBase extends Model {...}
class ModelTrait {...}
class User extends Mixin(ModelBase, ModelTrait) {...}

this line will throw error: https://github.com/Vincit/objection.js/blob/master/lib/model/Model.js#L866. The reason is because for Mixin, somehow the modelType.prototype can be undefined in this case, which throws 'Cannot convert null or undefined to object' error.

An easy fix is to add one check here (https://github.com/Vincit/objection.js/blob/master/lib/model/Model.js#L861) so we have:

if (modelClass === Model || modelClass.prototype == undefined) {
    // Stop recursion to the model class or its prototype is null or undefined
    return [];
  }

Is this a reasonable ask?