title: prefs.getString telling me that a String is a String?
body: <p>I am trying to save a string to shared preferences and then retrieve it.</p>
  <p>However, my Android Studio tells me that there is an error.</p>
  <p>Specifically, it says:</p>
  <blockquote>
  <p>The argument type 'String?' can't be assigned to the parameter type 'String'. However, I don't know what it is referring to as I don't think I ever specify that the variable is a String?.</p>
  </blockquote>
  <p>Here is the code:</p>
  <pre><code>  void _setFirstAppLaunchDate(DateTime value) async{
      SharedPreferences prefs = await SharedPreferences.getInstance();
      prefs.setString('firstLaunchDate', value.toString());
    }
  
    Future&lt;DateTime&gt; getFirstAppLaunchDate() async{
      SharedPreferences prefs = await SharedPreferences.getInstance();
      if (prefs.getString('firstLaunchDate') != null)
        return DateTime.parse(prefs.getString('firstLaunchDate'));
      else {
        var now = DateTime.now();
        _setFirstAppLaunchDate(now);
        return now;
      }
  </code></pre>