message: add null guard for crawling internal models when loading cards
diff: @@ -449,6 +449,7 @@ module.exports = class DocumentContext {
  }
  } else {
  let related = await this.read(linkage.type, linkage.id);
  + if (!related) { continue; }
  models.push(related);
  await this.crawlInternalModels(related, models);
  }
  