Another temporary fix for trees being unavailable after install, the context is null during upgrades in the installer, so would cause a YSOD

src/Umbraco.Web/umbraco.presentation/install/steps/theend.ascx.cs

@@ -32,7 +32,7 @@ namespace umbraco.presentation.install.steps
             if (!cms.businesslogic.skinning.Skinning.IsStarterKitInstalled())
                 customizeSite.Visible = false;
 
-            var initTrees = new ApplicationTreeRegistrar();
+            new ApplicationTreeRegistrar();
         }
 
         #region Web Form Designer generated code

src/umbraco.businesslogic/ApplicationTreeRegistrar.cs

@@ -31,7 +31,7 @@ namespace umbraco.BusinessLogic
         public ApplicationTreeRegistrar()
         {
 			//don't do anything if the application is not configured!
-			if (!ApplicationContext.Current.IsConfigured)
+			if (ApplicationContext.Current == null || !ApplicationContext.Current.IsConfigured)
 				return;
 
             // Load all Trees by attribute and add them to the XML config