dont' set applicationcontext to null on dispose for request based lifespan instances since applicationcontext is a singleton lifespan.

src/Umbraco.Web/Security/WebSecurity.cs

@@ -462,7 +462,6 @@ namespace Umbraco.Web.Security
         protected override void DisposeResources()
         {
             _httpContext = null;
-            _applicationContext = null;
         }
 
         

src/Umbraco.Web/UmbracoContext.cs

@@ -462,8 +462,6 @@ namespace Umbraco.Web
             Security.DisposeIfDisposable();
             Security = null;
             _umbracoContext = null;
-            //ensure not to dispose this!
-            Application = null;
 
             //Before we set these to null but in fact these are application lifespan singletons so 
             //there's no reason we need to set them to null and this also caused a problem with packages
@@ -471,7 +469,8 @@ namespace Umbraco.Web
             //http://issues.umbraco.org/issue/U4-2734
             //http://our.umbraco.org/projects/developer-tools/301-url-tracker/version-2/44327-Issues-with-URL-Tracker-in-614
             //ContentCache = null;
-            //MediaCache = null;     
+            //MediaCache = null;                 
+            //Application = null;
         }
     }
 }