title: Having problem while listening to stream of array of Objects comes from firestore collection
body: <p>I’m trying to read a collection from Firestore in my flutter app. Basically I get the collection snapshot and map it to my dart object. but when I listen to stream or use the stream in StreamProvider or StreamBuilder the snapshot data always is null!</p>
  
  <p>While debugging I noticed that data received as json and mapped correctly and result is cast to list so my stream should contains the correct data but on listen data is still sadly null.</p>
  
  <pre class="lang-dart prettyprint-override"><code>var ref = _db.collection('routes');
      final standardSerializers =
          (serializers.toBuilder()..addPlugin(StandardJsonPlugin())).build();
  
      ref.snapshots().map((list) {
        list.documents.map((doc) {
          print("Json:   " + doc.data.toString());
          print("Object:   " +
              standardSerializers
                  .deserializeWith(ClimbingRoute.serializer, doc.data)
                  .toString());
  
          return standardSerializers.deserializeWith(
              ClimbingRoute.serializer, doc.data);
        }).toList();
      }).listen((onData) {
        print("On listen: " + onData.toString());
      });
  </code></pre>
  
  <p>The console output:</p>
  
  <pre><code>00:21:31.055 69 info flutter.tools I/flutter (13897): Json:   {grade: 4c, gradingStyle: french, belayingStyle: auto, closure: flash, outCome: success, tags: []}
  00:21:31.056 70 info flutter.tools I/flutter (13897): Object:   ClimbingRoute {
  00:21:31.056 71 info flutter.tools I/flutter (13897):   outCome=success,
  00:21:31.056 72 info flutter.tools I/flutter (13897):   gradingStyle=french,00:21:31.056 73 info flutter.tools I/flutter (13897):   grade=4c,00:21:31.056 74 info flutter.tools I/flutter (13897):   belayingStyle=auto,00:21:31.056 75 info flutter.tools I/flutter (13897):   closure=flash,
  00:21:31.056 76 info flutter.tools I/flutter (13897):   tags={},
  00:21:31.056 77 info flutter.tools I/flutter (13897): }00:21:31.056 78 info flutter.tools I/flutter (13897): On listen: null
  </code></pre>