title: How to fix 'getter "documents" was called on null.' in flutter
body: <p>I am using flutter and firebase to create a mobile app. I have 2 collections on my firestore and i wanna read all documents in a the collection "posts". However, when I do that, an error saying the getter "documents" was called on null.</p>
  
  <pre><code>    Widget getContent(BuildContext context) {
        return StreamBuilder&lt;QuerySnapshot&gt;(
          stream: Firestore.instance.collection("posts").snapshots(),
          builder: (context, snap) {
            return CarouselSlider(
              enlargeCenterPage: true,
              height: MediaQuery.of(context).size.height,
              items: getItems(context, snap.data.documents),
          );
          },
          );
      }
  
      List&lt;Widget&gt; getItems(BuildContext context, List&lt;DocumentSnapshot&gt; 
      docs){
        return docs.map(
          (doc) {
            String content = doc.data["content"];
            return Text(content);
          }
        ).toList();
      }
  </code></pre>
  
  <p>I expected to be delivered with the data in all documents, nut instead got this error:</p>
  
  <pre><code>I/flutter (30878): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
  I/flutter (30878): The following NoSuchMethodError was thrown building StreamBuilder&lt;QuerySnapshot&gt;(dirty,
  I/flutter (30878): dependencies: [MediaQuery], state: _StreamBuilderBaseState&lt;QuerySnapshot,
  I/flutter (30878): AsyncSnapshot&lt;QuerySnapshot&gt;&gt;#72d38):
  I/flutter (30878): The getter 'documents' was called on null.
  I/flutter (30878): Receiver: null
  I/flutter (30878): Tried calling: documents
  I/flutter (30878): When the exception was thrown, this was the stack:
  V/NativeCrypto(30878): Registering com/google/android/gms/org/conscrypt/NativeCrypto's 284 native methods...
  I/flutter (30878): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
  I/flutter (30878): #1      PostsPageState.getContent.&lt;anonymous closure&gt;
  package:reach_out_kef_global/main.dart:140
  I/flutter (30878): #2      StreamBuilder.build 
  package:flutter/…/widgets/async.dart:423
  I/flutter (30878): #3      _StreamBuilderBaseState.build 
  package:flutter/…/widgets/async.dart:125
  I/flutter (30878): #4      StatefulElement.build 
  package:flutter/…/widgets/framework.dart:3825
  I/flutter (30878): #5      ComponentElement.performRebuild 
  package:flutter/…/widgets/framework.dart:3736
  I/flutter (30878): #6      Element.rebuild 
  package:flutter/…/widgets/framework.dart:3559
  I/flutter (30878): #7      ComponentElement._firstBuild 
  package:flutter/…/widgets/framework.dart:3716
  I/flutter (30878): #8      StatefulElement._firstBuild 
  package:flutter/…/widgets/framework.dart:3864
  I/flutter (30878): #9      ComponentElement.mount 
  package:flutter/…/widgets/framework.dart:3711
  I/flutter (30878): #10     Element.inflateWidget 
  package:flutter/…/widgets/framework.dart:2956
  I/flutter (30878): #11     Element.updateChild 
  package:flutter/…/widgets/framework.dart:2759
  I/flutter (30878): #12     ComponentElement.performRebuild 
  package:flutter/…/widgets/framework.dart:3747
  I/flutter (30878): #13     Element.rebuild 
  package:flutter/…/widgets/framework.dart:3559
  I/flutter (30878): #14     ComponentElement._firstBuild 
  package:flutter/…/widgets/framework.dart:3716
  I/flutter (30878): #15     StatefulElement._firstBuild 
  package:flutter/…/widgets/framework.dart:3864
  I/flutter (30878): #16     ComponentElement.mount 
  package:flutter/…/widgets/framework.dart:3711
  ...
  </code></pre>
  
  <p>PLEASE HELP!</p>