NullReferenceException - TaskStatus [Sequential SharePoint Workflow]

I am creating a SharePoint sequential workflow using a task. When I am running the workflow, I am able to go to the task list on item creation and then change the status to "Completed". After changing the status and saving it. I am facing a NullReferenceException for

string taskStatus = task1AfterProperties.ExtendedProperties[TaskStatusFieldId].ToString();

For declaring the TaskStatusFieldId, I have used these two GUIDs.

publicGuidTaskStatusFieldId=newGuid("c15b34c3-ce7d-490a-b133-3f4de8801b76");publicGuidTaskStatusFieldId=newGuid("73A146C9-1DF7-4FC1-8E9E-4E7F412F6E3B");

Even then I am facing the same exception. Can any one please help me out as where I made a mistake and how to resolve it.