Do not render survey pie chart which have not response

lib/models/survey.dart

@@ -33,6 +33,9 @@ class Survey {
 
   bool get hasResponded => this.userChoice != null && this.userChoice > 0;
 
+  bool get hasResponses =>
+      this.choices.where((f) => f.responses > 0).length > 0;
+
   SurveyChoice get userResponse {
     if (!hasResponded) return null;
 

lib/ui/widgets/survey_widget.dart

@@ -42,9 +42,7 @@ class _SurveyWidgetState extends State<SurveyWidget> {
           style: TextStyle(fontWeight: FontWeight.bold),
         ),
         _buildUserResponse(),
-        PieChart(
+        survey.hasResponses ? _buildChart() : _buildNoResponseNotice(),
-          dataMap: _buildDataMap(),
-        ),
       ],
     );
   }
@@ -98,15 +96,28 @@ class _SurveyWidgetState extends State<SurveyWidget> {
       items: survey.choices
           .map(
             (f) => DropdownMenuItem<SurveyChoice>(
-                  value: f,
+              value: f,
-                  child: Text(f.name),
+              child: Text(f.name),
-                ),
+            ),
           )
           .toList(),
       onChanged: _respondToSurvey,
     );
   }
 
+  Widget _buildChart() {
+    return PieChart(
+      dataMap: _buildDataMap(),
+    );
+  }
+
+  Widget _buildNoResponseNotice() {
+    return Padding(
+      padding: const EdgeInsets.all(16.0),
+      child: Text(tr("No response yet to this survey.")),
+    );
+  }
+
   /// Respond to survey
   Future<void> _respondToSurvey(SurveyChoice choice) async {
     // Send the response to the server