The messages are not displayed

**Describe the bug**
Build the demo _Cloud Functions for Firebase_.
After completing the deploy, try to send the message, but no message displayed.

Errors in the console:
```
Uncaught TypeError: timestamp.toMillis is not a function
at createAndInsertMessage (main.js:265)
at displayMessage (main.js:299)
at main.js:77
at Array.forEach ()
at Object.next (main.js:72)
at next (database.ts:1825)
at async_observer.ts:51
```

Firebase-tools version: 7.7.0

**To Reproduce**
Steps to reproduce the behavior:
1. Deploy the project by following the sample _Cloud Functions for Firebase_
2. Visit the page, input the message then send

**Expected behavior**
The message is displayed on the page.

**Additional context**