fix issue of main.dart

Author: Zuoweixiang

example/lib/main.dart

@@ -51,6 +51,10 @@ class _MyAppState extends State<MyApp> {
         ),
         body: ListView(
           children: <Widget>[
+            ListTile(
+              title: Text(_distinctId),
+              onTap: (){},
+            ),
             ListTile(
               title: Text('This is the official Flutter Plugin for Sensors Analytics.'),
               onTap: (){},