title: Flutter mockito with null safety throws Null type error
body: <p>I've been having this issue for a while following the official docs on mocking.</p>
  <p>This test when run throws an error <code>type 'Null' is not a subtype of type 'String'</code></p>
  <p><a href="https://i.stack.imgur.com/KYXvx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KYXvx.png" alt="Test with error" /></a></p>
  <p>When I change from <code>String</code> to <code>String?</code> on <code>getString()</code> it passes and I don't understand why. I have no intention for the function to ever be null so I don't need to make it nullable.</p>