Fixed issue in semantic highlighting type errors

ICSharpCode.NRefactory.CSharp/Analysis/SemanticHighlightingVisitor.cs

@@ -514,6 +514,10 @@ namespace ICSharpCode.NRefactory.CSharp.Analysis
 				case TypeKind.Delegate:
 					color = delegateTypeColor;
 					return true;
+				case TypeKind.Unknown:
+				case TypeKind.Null:
+					color = syntaxErrorColor;
+					return true;
 				default:
 					color = default (TColor);
 					return false;

ICSharpCode.NRefactory.Tests/CSharp/Analysis/SemanticHighlightingTests.cs

@@ -303,6 +303,22 @@ class Class {
 			TestColor (code, syntaxErrorColor);
 		}
 
+		[Test]
+		public void TestNullTypeError()
+		{
+			string code =@"
+		class A
+		{
+			public static void Main ()
+			{
+				$var a = null;
+			}
+		}
+";
+			TestColor (code, syntaxErrorColor);
+		}
+
+
 
 
 	}