App: Pages: Add a null check in OnSelectTemplate() (#243)

To avoid accessing a null poiter add a null check in OnSelectTemplate().

Signed-off-by: Alistair Francis <alistair@alistair23.me>

src/App/Pages/Vault/VaultListGroupingsPage.cs

@@ -350,6 +350,10 @@ namespace Bit.App.Pages
 
             protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
             {
+                if(item == null)
+                {
+                    return null;
+                }
                 return ((GroupingOrCipher)item).Cipher == null ? GroupingTemplate : CipherTemplate;
             }
         }