fix NRE in TypeGuessing after `yield return new`

src/Libraries/NRefactory/ICSharpCode.NRefactory.CSharp/Refactoring/TypeGuessing.cs

@@ -259,9 +259,20 @@ namespace ICSharpCode.NRefactory.CSharp
 			}
 
 			if (expr.Parent is YieldReturnStatement) {
-				var state = resolver.GetResolverStateBefore(expr);
-				if (state != null && (state.CurrentMember.ReturnType is ParameterizedType)) {
-					var pt = (ParameterizedType)state.CurrentMember.ReturnType;
+				ParameterizedType pt = null;
+				var parent = expr.Ancestors.FirstOrDefault(n => n is EntityDeclaration || n is AnonymousMethodExpression|| n is LambdaExpression);
+				if (parent != null) {
+					var rr = resolver.Resolve(parent);
+					if (!rr.IsError)
+						pt = rr.Type as ParameterizedType;
+				}
+				var e = parent as EntityDeclaration;
+				if (e != null) {
+					var rt = resolver.Resolve(e.ReturnType);
+					if (!rt.IsError)
+						pt = rt.Type as ParameterizedType;
+				}
+				if (pt != null) {
 					if (pt.FullName == "System.Collections.Generic.IEnumerable") {
 						return new [] { pt.TypeArguments.First() };
 					}