Fix #613: NullReferenceException when I add `new DirectoryInfo(".")` to watches in debug.

src/AddIns/Debugger/Debugger.AddIn/NRefactory/ExpressionEvaluationVisitor.cs

@@ -444,7 +444,7 @@ namespace Debugger.AddIn
 			} else
 				throw new GetValueException("Invoked member must be a method or property");
 			Value target = null;
-			if (!usedMethod.IsStatic)
+			if (!usedMethod.IsStatic && !usedMethod.IsConstructor)
 				target = Convert(result.TargetResult).GetPermanentReference(evalThread);
 			return InvokeMethod(target, usedMethod, result.Arguments.Select(rr => Convert(rr).GetPermanentReference(evalThread)).ToArray());
 		}

src/AddIns/Debugger/Debugger.Core/Value.cs

@@ -416,6 +416,8 @@ namespace Debugger
 			if (memberInfo == null)
 				throw new DebuggerException("memberInfo is null");
 			if (!memberInfo.IsStatic) {
+				if (memberInfo.SymbolKind == SymbolKind.Constructor)
+					throw new GetValueException("Constructor expression");
 				if (objectInstance == null)
 					throw new DebuggerException("No target object specified");
 				if (objectInstance.IsNull)