Description of the problem:

My App aim is to store family spending in Firebase Realtime Database. I want that, when a new spending is stored, a notification is sent to all other devices. I try to send a notification to a single device and it works fine, but when I try to get all the tokens in an array, I have an error: TypeError: Cannot convert undefined or null to object at Function.keys ().

code of index.js :

const functions = require("firebase-functions");
const admin = require("firebase-admin");
 admin.initializeApp();

exports.androidPushNotification = functions.database
    .ref("{nodo}/spese/{spesaPush}")
    .onCreate(async (snapshot, context) => {
      const original = snapshot.val();

      const getDeviceTokensPromise = admin.database()
          .ref(`/utenti/{tutti}/token`).once('value');

      let tokensSnapshot;
      let tokens;
      tokensSnapshot = await getDeviceTokensPromise;

      // This is the line generating the errors. 
      // If I get only a specific token and 
      // use tokens = tokensSnapshot.val(); anything works fine
      tokens = Object.keys(tokensSnapshot.val());


      const result = original.chi + " ha speso " +
      original.costo + " € per acquistare " +
      original.desc;
      console.log(result);

      const payload = {
        notification: {
          title: 'New spending inserted!',
          body: result, 
        }
      };

      const response = await admin.messaging().sendToDevice(tokens, payload);
      return result;
    });

It seems that values are not reached yet, but I thought that the keyword await lets system to wait until all data are available. From the log I noticed that the value I need is null and I don't understand why. If I use this line of code:

      const getDeviceTokensPromise = admin.database()
          .ref(`/utenti/SpecificUser/token`).once('value');
      ....
      //then
      tokens = tokensSnapshot.val();

The notification is sent to the device that has the token under the name of "SpecificUser"

EDIT: I provide a pic of my db. I notice that none of the field is null, so I don't know why I see this error

Thank you to anyone that helps me