title: What is onUnknownRoute in flutter
body: <p>I'm new to flutter, In MaterailApp Widget have an attribute called <a href="https://api.flutter.dev/flutter/material/MaterialApp/onUnknownRoute.html" rel="noreferrer">onUnknownRoute</a>. What is the main use of onUnknownRoute?</p>
  
  <p>Thanks for your help!</p>