title: The argument type 'DateTime?' can't be assigned to the parameter type 'DateTime'
body: <p>In Book : Flutter Apprentice First Edition, I had below code:</p>
  <pre><code>import 'package:flutter/painting.dart';
  // 1
  enum Importance { low, medium, high }
  class GroceryItem {
  // 2
  final String id;
  // 3
  final String name;
  final Importance importance;
  final Color color;
  final int quantity;
  final DateTime date;
  final bool isComplete;
  ...
  </code></pre>
  <p>And the build method as</p>
  <pre><code>Widget buildDate() {
      final dateFormatter = DateFormat('MMMM dd h:mm a');
      final dateString = dateFormatter.format(item!.date);
      return Text(
        dateString,
        style: TextStyle(decoration: textDecoration),
      );
    }
  </code></pre>
  <p>I have the below error:</p>
  <blockquote>
  <p>The argument type 'DateTime?' can't be assigned to the parameter type 'DateTime'.</p>
  </blockquote>