Added a null check to IUmbracoHelperAccessor & IPublishedSnapshotAccessor

src/Umbraco.Core/PublishedCache/IPublishedSnapshotAccessor.cs

@@ -1,10 +1,10 @@
-namespace Umbraco.Cms.Core.PublishedCache
+namespace Umbraco.Cms.Core.PublishedCache
 {
     /// <summary>
     /// Provides access to the "current" <see cref="IPublishedSnapshot"/>.
     /// </summary>
     public interface IPublishedSnapshotAccessor
     {
-        IPublishedSnapshot PublishedSnapshot { get; set; }
+        bool TryGetPublishedSnapshot(out IPublishedSnapshot publishedSnapshot);
     }
 }