title: When I use Promise with dialogflow library in server - get Error
body: <p>I try to execute this code on Firebase Cloud Functions</p>
  
  <pre><code>const functions = require('firebase-functions');
  const admin = require('firebase-admin');
  const firebase_database = require('./conf/firebase');
  const { WebhookClient } = require('dialogflow-fulfillment');
  
  exports.dialogflowFirebaseFulfillment = functions.https.onRequest((request, response) =&gt; {
       function searcheColleagueByName(agent){
         var lastname = agent.parameters.lastname;        
         firebase_database.ref().once('value')
         .then(team =&gt; {
             agent.add("some name " + lastname);
          })
         .catch(err=&gt;{
             agent.add("something wrong");
         })
       }
  </code></pre>
  
  <p>and after when i make request to my bot from telegram I get error in firebase console:</p>
  
  <pre><code>Error: No responses defined for platform: null
      at V2Agent.sendResponses_ (/srv/node_modules/dialogflow-fulfillment/src/v2-agent.js:243:13)
      at WebhookClient.send_ (/srv/node_modules/dialogflow-fulfillment/src/dialogflow-fulfillment.js:505:17)
      at promise.then (/srv/node_modules/dialogflow-fulfillment/src/dialogflow-fulfillment.js:316:38)
      at &lt;anonymous&gt;
      at process._tickDomainCallback (internal/process/next_tick.js:228:7)
  </code></pre>
  
  <p>What's wrong? why when I use <code>promisse</code> my agent can't give me an answer?</p>