title: Uncaught TypeError: Cannot set properties of null (setting 'onclick')?
body: <p>I wrote a very simple html css program and add a javascript in it, here is my program and my javascript in it</p>
  <p>(my program : <a href="https://github.com/phianh2904/phan-8-web-the-band-tablet-responsive" rel="nofollow noreferrer">https://github.com/phianh2904/phan-8-web-the-band-tablet-responsive</a>)</p>
  <p>my javacript in index.html file</p>
  <pre><code>    &lt;script&gt;
      var header = document.getElementById('header');
      var mobileMenu = document.getElementById('mobile-menu');
  
      mobileMenu.onclick = function() {
          console.log(header.clientHeight);
      }
  
      const buyBtns = document.querySelectorAll('.js-buy-ticket')
      const modal = document.querySelector('.js-modal')
      const modalclose = document.querySelector('.js-modal-close')
      const modalcontainer = document.querySelector('.js-modal-container')
  
      // ham hien thi modal mua ve tuc la them class open vao modal
      function showBuyTickets() {
          modal.classList.add('open')
      }
  
      // ham an modal mua ve them class open vao modal
      function hideBuyTickets() {
          modal.classList.remove('open')
      }
  
  
      // lap qua tung the button va nghe hanh vi click
      for (const buyBtn of buyBtns) {
          buyBtn.addEventListener('click', showBuyTickets)
      }
  
      // nghe hanh vi click vao button close
      modalclose.addEventListener('click', hideBuyTickets)
  
      //clcik vao khoang khong de dong cac the con lai
      modal.addEventListener('click', hideBuyTickets)
  
      //click vao ben trong thi khong bi lam sao ca
      modalcontainer.addEventListener('click', function(event) {
          event.stopPropagation()
      })
      &lt;/script&gt;
  </code></pre>
  <p>As you can see in my javascript code snippet, it is very simple. I Wrote the code</p>
  <pre><code>var header = document.getElementById('header');
  var mobileMenu = document.getElementById('mobile-menu');
  mobileMenu.onclick = function() {
        console.log(header.clientHeight);
  }
  </code></pre>
  <p>to show the clientHeight.</p>
  <p>But here is the error I had :</p>
  <p><a href="https://i.stack.imgur.com/mWIx9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mWIx9.png" alt="picture about my problem" /></a></p>
  <p>Uncaught TypeError: Cannot set properties of null (setting 'onclick')
  at index.html:231:28</p>
  <p>As they said in this link <a href="https://stackoverflow.com/questions/9778888/uncaught-typeerror-cannot-set-property-onclick-of-null">Uncaught TypeError: Cannot set property &#39;onclick&#39; of null</a></p>
  <p>I put the script in the end of the body element, and wrap code in  but it did not work.
  So could you please give me some answer for this problem ? Thank you very much for your time.</p>