title: getElementById returns null (in extension to modify website)
body: <p>I am currently trying to create a chrome <strong>extension</strong> <strong><em>that alters a website</em></strong> (an interface for a ip camera that I bought, thats also why the site is a local site like 192.168.178.70 for example).
  I just want to add some buttons and stuff like that to add some functionality.</p>
  
  <p>I used</p>
  
  <pre><code>document.body.onload
  </code></pre>
  
  <p>to wait until everything has loaded so I can grab a table or a div or whatever and create and add my button to it.
  But document.getElementById("ipcam_wrapper") does not seem to work properly, it returns null.</p>
  
  <p>Well I tried to do the same thing in google chrome developer mode but it does also return null. I actually managed to get the element this way once but I am not able to recreate it because I did not do anything different.</p>
  
  <p>There is only one element with that specific ID.</p>
  
  <p><a href="https://i.stack.imgur.com/MLI65.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MLI65.png" alt="Only one element with that ID"></a></p>
  
  <p>Here i try to get the element:</p>
  
  <p><a href="https://i.stack.imgur.com/cIBgg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cIBgg.png" alt="Trying to get the element"></a></p>
  
  <p>So what am I doing wrong?</p>
  
  <p>best regards</p>