Change Workflow Tasks list using visual studio designer in SharePoint state machine workflow

Question

I have created a state machine workflow using VS 2013,SP 2013.When i haev deployed to a document library, its thrown an err "

Error occured

when I debugged, am able to see the task list of this document library is showing NULL value. I am stuck here, with why i am getting a NULL value.While creating this workflow i already configured, selected the "workflowtasks" list as the associated task list[ NOT the normal Tasks list]. Am I doing wrong thing? by selecting this workflowtasks as the task list for this workflow? How to change the Task list of this workflow from tasks list to "Workflow Tasks" list using Visual Studio designer surface?

    private void createTaskTokenfoInProgressActi_MethodInvoking(object 
       sender, EventArgs e)
      {
        createTaskWithContentType1_TaskId1 = Guid.NewGuid();
        **//below line throws the error**
        if (this.workflowProperties.TaskList.ContentTypesEnabled != true)
        {
            workflowProperties.TaskList.ContentTypesEnabled = true;
        }

        strworkflowName = "Document approval workflow";

        //loggedinUser = workflowProperties.Web.CurrentUser.LoginName;            
        loggedDate = oSPListItem["InitiationDate"].ToString();            
          siteName = ospweeb.Title;  
             //workflowProperties.Site.ToString();
                SPContentTypeId myContentTypeID = new 
            SPContentTypeId(createTaskWithContentType1.ContentTypeId);
                 SPContentType myContentType = 
           workflowProperties.Site.RootWeb.ContentTypes[myContentTypeID];
            bool contenTypeExists = false;
          foreach (SPContentType contentType in 
          workflowProperties.TaskList.ContentTypes)
           {
             if (contentType.Name == myContentType.Name)
             {
                 contenTypeExists = true;
                break;
              }
          }

        // If it's not already added, then Add it to the Task Lists 
        Content Types

Answer 1

It may be because of the workflow tasks list might not have created properly. Are you creating your teamsite/publishing sub site programmatically / manually?

Have you initialized the oSPListItem object?