title: Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't
body: <p>Good day,
  I have been trying the below code:</p>
  <pre><code>import 'dart:io';
  main (){
  print (&quot;write your birth year:&quot;);
  var birthyear = stdin.readLineSync();
  var birthyearint = int.parse(birthyear);
  var age = 2021-birthyearint;
  print(age);
  }
  </code></pre>
  <p>when I run it I receive the following error:</p>
  <p>5:30: Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't.
  var birthyearint = int.parse(birthyear);
  ^</p>