title: StreamBuilder<QuerySnapshot>(dirty, state: _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>):
body: <p>I cant able to retrive the data from Firestore and getting Error as below,</p>
  <blockquote>
  <p>════════ Exception caught by widgets library
  ═══════════════════════════════════════════════════════ The following
  assertion was thrown building StreamBuilder(dirty,
  state: _StreamBuilderBaseState&lt;QuerySnapshot,
  AsyncSnapshot&gt;#e568b): A build function returned null.</p>
  <p>The offending widget is: StreamBuilder Build functions
  must never return null.</p>
  <p>To return an empty space that causes the building widget to fill
  available room, return &quot;Container()&quot;. To return an empty space that
  takes as little room as possible, return &quot;Container(width: 0.0,
  height: 0.0)&quot;.</p>
  <p>The relevant error-causing widget was:    StreamBuilder
  file:...dart:140:15 When the exception was thrown, this was the stack:</p>
  <p>#0      debugWidgetBuilderValue. (package:flutter/src/widgets/debug.dart:300:7)
  #1      _Closure.call (dart:core-patch/function.dart)
  #2      debugWidgetBuilderValue (package:flutter/src/widgets/debug.dart:321:4)
  #3      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4569:7)
  #4      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4737:11) ...</p>
  </blockquote>
  <hr />
  <p>Below is my code.</p>
  <pre><code>StreamBuilder&lt;QuerySnapshot&gt;(
                    stream: Firestore.instance.collection(&quot;currency&quot;).snapshots(),
                    builder: (context, snapshot) {
                    if (!snapshot.hasData){
                        print('test pharse');
                         Text(&quot;Loading.....&quot;);}
                      else {
                        List&lt;DropdownMenuItem&gt; currencyItems = [];
                        for (int i = 0; i &lt; snapshot.data.documents.length; i++) {
                          DocumentSnapshot snap = snapshot.data.documents[i];
                          currencyItems.add(
                            DropdownMenuItem(
                              child: Text(
                                snap.documentID,
                                style: TextStyle(color: Color(0xff11b719)),
                              ),
                              value: &quot;${snap.documentID}&quot;,
                            ),
                          );
                        }
                        return Row(
                          mainAxisAlignment: MainAxisAlignment.center,
                          children: &lt;Widget&gt;[
                            Icon(Icons.mail,
                                size: 25.0, color: Color(0xff11b719)),
                            SizedBox(width: 50.0),
                            DropdownButton(
                              items: currencyItems,
                              onChanged: (currencyValue) {
                                final snackBar = SnackBar(
                                  content: Text(
                                    'Selected Currency value is $currencyValue',
                                    style: TextStyle(color: Color(0xff11b719)),
                                  ),
                                );
                                Scaffold.of(context).showSnackBar(snackBar);
                                setState(() {
                                  selectedCurrency = currencyValue;
                                });
                              },
                              value: selectedCurrency,
                              isExpanded: false,
                              hint: new Text(
                                &quot;Choose Currency Type&quot;,
                                style: TextStyle(color: Color(0xff11b719)),
                              ),
                            ),
                          ],
                        );
                      }
                    }),
  </code></pre>