bug fix in cutomTweenDemo (#675)

animations/lib/src/basics/06_custom_tween.dart

@@ -71,6 +71,9 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
         ],
       ),
       body: SafeArea(
+        child: Container(
+          alignment: Alignment.bottomCenter,
+          child: SingleChildScrollView(
             child: Column(
               mainAxisAlignment: MainAxisAlignment.end,
               crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -94,6 +97,8 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
               ],
             ),
           ),
+        ),
+      ),
     );
   }
 }