Fixes up ResolverBase and it's static lock object which isn't actually required because

our singleton is a getter/setter and field access is thread safe. Updated SingleResolverBase
to ensure that TResolved is a class so the null check works properly.

src/Umbraco.Core/Resolving/ResolverBase.cs

@@ -3,36 +3,13 @@ using System.Threading;
 
 namespace Umbraco.Core.Resolving
 {
-	internal abstract class ResolverBase<TResolver> where TResolver : class
+	/// <summary>
+	/// base class for resolvers which declare a singleton accessor
+	/// </summary>
+	/// <typeparam name="TResolver"></typeparam>
+	internal abstract class ResolverBase<TResolver> 
+		where TResolver : class
 	{
-		static TResolver _resolver;
+		public static TResolver Current { get; set; }
-
-		//TODO: This is not correct, this will be the same lock for all ResolverBase classes!!
-		static readonly ReaderWriterLockSlim ResolversLock = new ReaderWriterLockSlim();
-
-		public static TResolver Current
-		{
-			get
-			{
-				using (new ReadLock(ResolversLock))
-				{
-					if (_resolver == null)
-						throw new InvalidOperationException("Current has not been initialized. You must initialize Current before trying to read it.");
-					return _resolver;
-				}
-			}
-
-			set
-			{
-				using (new WriteLock(ResolversLock))
-				{
-					if (value == null)
-						throw new ArgumentNullException("value");
-					if (_resolver != null)
-						throw new InvalidOperationException("Current has already been initialized. It is not possible to re-initialize Current once it has been initialized.");
-					_resolver = value;
-				}
-			}
-		}
 	}
 }

src/Umbraco.Core/Resolving/SingleResolverBase.cs

@@ -3,7 +3,8 @@
 namespace Umbraco.Core.Resolving
 {
 	internal abstract class SingleResolverBase<TResolver, TResolved> : ResolverBase<TResolver> 
-		where TResolver : class
+		where TResolver : class 
+		where TResolved : class
 	{
 		TResolved _resolved;
 		readonly bool _canBeNull;