```js
 function w3_open() {
 document.getElementById("mySidebar").style.display = "block";
 document.getElementById("w3-button").style.display = "none"
 document.getElementById("w3-buttondos").style.display ="inline-block"

 }

 function w3_close() {
 document.getElementById("mySidebar").style.display = "none";
 document.getElementById("w3-buttondos ").style.display = "none"
 document.getElementById("w3-button").style.display ="inline-block"
 }
```

```html
<button class="w3-button w3-teal" onclick="w3_open()">&#9776;</button>
     <button class="w3-buttondos w3-teal" onclick="w3_close()">&#10006;</button>
```

it says me -> Uncaught TypeError: document.getElementById(...) is null