Firebase部署失败，并显示“每个都应返回值或抛出承诺”

Question

'use-strict'

 const functions = require('firebase-functions');

 const admin=require('firebase-admin');

    admin.initializeApp(functions.config().firebase);


  exports.pushNotification=functions.firestore.
  document("Users/{user_id}/Notification/{notification_Id}").onWrite((change, 
  context)=> {
  const user_id=context.params.user_id;
  const notification_Id=context.params.notification_Id;

   return admin.firestore().collection("Users").doc(user_id)
  .collection("Notification").doc(notification_id).get().then(queryResult => {
const from_user_id=queryResult.data().from;
const from_data=admin.firestore().collection("Users").doc(from_user_id).get();
const to_data=admin.firestore().collection("Users").doc(user_id).get();

  return Promise.all([from_data,to_data]).then(result => {
  const from_email=result[0].data().email;
  const to_email=result[1].data().email;
  console.log("from " + from_email + " to " + to_email);

});

 });

});

它有1条警告和1条错误。

警告是避免嵌套承诺。

错误是（然后每个错误都应返回值或抛出诺言/始终返回）..........