sample: [StateViewer]右クリックメニューが正しく解放されない不具合を修正

SamplePlugins/VehiclePlugins/StateViewer/StateViewer.cs

@@ -36,6 +36,7 @@ public StateViewer(PluginBuilder services) : base(services)
         public override void Dispose()
         {
             Form.Close();
+            MenuItem.Dispose();
         }
 
         public override TickResult Tick(TimeSpan elapsed)