title: Flutter build web
body: <p>I'm new in flutter and I want to build web application by flutter web,<code>flutter build apk</code> is worked for me, but when I run <code>Flutter build web</code>
  <a href="https://i.stack.imgur.com/zExwr.png" rel="nofollow noreferrer">I have this error.</a></p>
  <p>I followed up every step  according to <a href="https://dart.dev/null-safety/unsound-null-safety" rel="nofollow noreferrer">this document</a> and use <code>// @dart=2.9</code> top of all of my class that has error null check.</p>
  <p>do you have any idea about this problem?!</p>