Updated StoreInterop class and ads appearing conditions

FoxTube.Core/Helpers.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace FoxTube.Core
+{
+    public static class Helpers
+    {
+        public static Uri ToUri(this string str) =>
+            string.IsNullOrWhiteSpace(str) ? null : new Uri(str);
+    }
+}