title: "Use key in widget constructors" warning each time I create a new widget
body: <p>I've started getting a warning every time I create a new class.</p>
  <p><a href="https://i.stack.imgur.com/FPyBT.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FPyBT.png" alt="enter image description here" /></a></p>
  <blockquote>
  <p>Use key in widget constructors.</p>
  </blockquote>
  <blockquote>
  <p>Prefer const with constant constructor.</p>
  </blockquote>
  <p>and so on. Where these errors are coming from and how do I get rid of it?</p>