title: self.root.ids is empty even after giving ids in kivyMD
body: <p>I am trying to create a nice login screen with kivyMD. I am Using Screen Manager to handle different screens. Also I am using MDToolbar and MDBottomNavigationItem to create different screens.</p>
  <p>The problem is the front end is working well. But, when I assign id to a textfield or a label, it is not added to self.root.ids. I tried to print self.root.ids but it was an empty dictionary.</p>
  <p>From Python:</p>
  <pre><code>print(self.root.ids)
  </code></pre>
  <p>gives an empty dictionary {}.</p>
  <p>From KV:</p>
  <pre><code>print(app.root.ids)
  </code></pre>
  <p>Same output {}.</p>
  <p>I can't get the reason and also, I faced some errors like <code>__getattr__</code> attribute errors while experimenting with the code to solve the problem.</p>
  <p>This is how I am assigining id to a text field:</p>
  <pre><code>MDTextField:
      id: username
  </code></pre>
  <p>Thanks for the help.</p>