title: Firebase admin SDK FCM error
body: <p>I am using version 6.0.0 of firebase admin in Dialogflow fulfilments to send fcm notification. </p>
  
  <pre><code>admin.messaging().send({
      "message": {
          "notification": {
              "title": "Portugal vs. Denmark",
              "body": "great match!"
          },
          "token": user_info.device_id
      }
  })
  .then((response) =&gt; {
      // Response is a message ID string.
      console.log('Successfully sent message:', response);
  })
  .catch((error) =&gt; {
      console.log('Error sending message:', error);
  });
  </code></pre>
  
  <p>This is the error I am getting </p>
  
  <pre><code>Error: Exactly one of topic, token or condition is required
  at FirebaseMessagingError.Error (native)
  at FirebaseMessagingError.FirebaseError [as constructor] (/user_code/node_modules/firebase-admin/lib/utils/error.js:39:28)
  at FirebaseMessagingError.PrefixedFirebaseError [as constructor] (/user_code/node_modules/firebase-admin/lib/utils/error.js:85:28)
  at new FirebaseMessagingError (/user_code/node_modules/firebase-admin/lib/utils/error.js:241:16)
  at validateMessage (/user_code/node_modules/firebase-admin/lib/messaging/messaging.js:320:15)
  at Messaging.send (/user_code/node_modules/firebase-admin/lib/messaging/messaging.js:461:9)
  at snapshot.forEach (/user_code/index.js:130:19)
  at /user_code/node_modules/firebase-admin/node_modules/@firebase/database/dist/index.node.cjs.js:4255:20
  at /user_code/node_modules/firebase-admin/node_modules/@firebase/database/dist/index.node.cjs.js:3669:24
  at LLRBNode.inorderTraversal (/user_code/node_modules/firebase-admin/node_modules/@firebase/database/dist/index.node.cjs.js:2606:13)
  </code></pre>