title: Flutter 2.0.4 (stable): Null check operator used on a null value ERROR
body: <p>I recently migrated my app to Flutter 2.0 which contains Null safety. After successful migration, when I run the app, I get the following error</p>
  <p><em>The following _CastError was thrown building ContactListWidget(dirty, dependencies: [MediaQuery, _InheritedProviderScope], state: _ContactListWidgetState#9ca7f)</em></p>
  <p>The error mentions no line number where the error is getting thrown, the code is approximately 500 lines :/</p>
  <p>How do I fix this issue? Is it related to Firebase queries/data?</p>
  <p><strong>EDIT</strong></p>
  <p>I was able to locate the erroneous line, and it seems like a problem with the usage of <code>?</code>  and <code>!</code>. Can anyone direct to a resource where it is easy to understand null safety? Thanks</p>