message: Fix toObject method of TaskUnit
diff: @@ -26,11 +26,11 @@ const taskSchema = new mongoose.Schema(
  timestamps: true,
  toObject: {
  transform: function (doc: any, ret: any) {
  - if (ret.hasOwnProperty('_id')) {
  + if (ret.hasOwnProperty('_id') && ret._id !== null) {
  ret._id = ret._id.toString();
  }
  - if (ret.hasOwnProperty('id')) {
  + if (ret.hasOwnProperty('id') && ret.id !== null) {
  ret.id = ret.id.toString();
  }
  ret.answers = ret.answers.map((answer: any) => {
  