message: fixing crash on mobiledoc when sourceField is missing
diff: @@ -7,6 +7,11 @@ exports.type = '@cardstack/core-types::integer';
  exports.compute = async function(model, { sourceField }) {
  let mobiledoc = await model.getField(sourceField);
  +
  + if (!mobiledoc) {
  + return 0;
  + }
  +
  let bodyText = renderer.render(mobiledoc).result;
  return Math.round(bodyText.split(' ').length / WORDS_PER_MIN);
  