Fix ArgumentNullException when using Expression and OrbitGenerator Be…

…haviour together (#35)

The latter creates AlwaysTrue parameter which doesn't have an ID

source/ContractConfigurator/Behaviour/Expression.cs

@@ -123,7 +123,7 @@ protected override void OnCompleted()
 
         protected override void OnParameterStateChange(ContractParameter param)
         {
-            if (param.State == ParameterState.Complete && onParameterComplete.ContainsKey(param.ID))
+            if (param.ID != null && param.State == ParameterState.Complete && onParameterComplete.ContainsKey(param.ID))
             {
                 ExecuteExpressions(onParameterComplete[param.ID]);
                 onParameterComplete[param.ID].Clear();