Fixed a crash on android

src/HotUI.Android/Handlers/VStackHandler.cs

@@ -82,9 +82,11 @@ namespace HotUI.Android.Handlers
             foreach (var child in children)
             {
                 var cview = child.ToView();
-                views.Add(cview);
-                //cview.ContentMode = UIViewContentMode.Top;
-                base.AddView(cview);
+                if (cview != null)
+                {
+                    views.Add(cview);
+                    base.AddView(cview);
+                }
             }
         }
     }

src/HotUI.Android/UI.cs

@@ -6,32 +6,39 @@ namespace HotUI.Android
 {
     public static class UI
     {
-        static bool hasInit;
+        static bool _hasInitialized;
 
         public static void Init()
         {
-            if (hasInit)
-                return;
-            hasInit = true;
+            if (_hasInitialized) return;
+            _hasInitialized = true;
+
+            // Controls
             Registrar.Handlers.Register<Button, ButtonHandler>();
-            Registrar.Handlers.Register<Toggle, ToggleHandler>();
+            Registrar.Handlers.Register<Image, ImageHandler>();
             Registrar.Handlers.Register<TextField, TextFieldHandler>();
             Registrar.Handlers.Register<Text, TextHandler>();
-            Registrar.Handlers.Register<VStack, VStackHandler>();
-            Registrar.Handlers.Register<HStack, HStackHandler>();
+            Registrar.Handlers.Register<Toggle, ToggleHandler>();
             //Registrar.Handlers.Register<WebView, WebViewHandler> ();
+
+            // Containers
             Registrar.Handlers.Register<ScrollView, ScrollViewHandler>();
-			Registrar.Handlers.Register<Image, ImageHandler> ();
 			Registrar.Handlers.Register<ListView, ListViewHandler> ();
 			Registrar.Handlers.Register<View, ViewHandler>();
             Registrar.Handlers.Register<ContentView, ContentViewHandler>();
             Registrar.Handlers.Register<ViewRepresentable, ViewRepresentableHandler>();
 
-            Device.PerformInvokeOnMainThread = (a) => AndroidContext.CurrentContext.RunOnUiThread(a);
-            Device.GraphicsService = new AndroidGraphicsService();
+            // Native Layouts
+            Registrar.Handlers.Register<HStack, HStackHandler>();
+            Registrar.Handlers.Register<VStack, VStackHandler>();
+
+            // Modal SUpport
             ModalView.PerformPresent = ModalManager.ShowModal;
             ModalView.PerformDismiss = ModalManager.DismisModal;
 
+            // Device Services
+            Device.PerformInvokeOnMainThread = (a) => AndroidContext.CurrentContext.RunOnUiThread(a);
+            Device.GraphicsService = new AndroidGraphicsService();
         }
     }
 }