Error : firebase cloud funtion, firebase database - firebase

Question

when i run this code in firebase cloud function, it got error "Registration token(s) provided to sendToDevice() must be a non-empty string or a non-empty array." i know that the problems happen because the function return before the tokens haven been loaded then i use promises but didn't work.
.ref('/alertToUser/{userId}/{timestamp}')
.onCreate((snapshot, context) =>
{
const userId=context.params.userId
const userName=snapshot.val().userName
const timestamp=context.params.timestamp
const message=snapshot.val().message;
let promises = [];
var DeviceToken = [];
const payload = {
"data":{
"topic":"alertToUser",
"message" : message,
"timestamp" : `${timestamp}`,
"userName" : userName
}
};
const options = {
"priority":"high"
};
let promise=admin.database().ref(`alertToUser/${userId}/${timestamp}/recieverID`).once("value",(snapshot) => {
snapshot.forEach((userSnap) => {
var recieverID=userSnap.val();
admin.database().ref(`/User/${recieverID}/token`).once("value",(snapshot)=>{
var token= snapshot.val();
DeviceToken.push(token);
console.log(`token: ${DeviceToken}`);
})
});
return true;
}
)
promises.push(promise);
return Promise.all(promises).then(() => {return admin.messaging().sendToDevice(DeviceToken, payload,options);});
});