Project Server 2010 Event Handler

I am working on OnPublished event handler that will update one custom field of a project based on change in another field.

I am getting an error

Event Handler for event \ProjectPublished\ of type \PS.UpdateProjectStatusChangeDate.EventHandlerUpdateField\ threw an exception: ProjectServerError(s) LastError=CICOCheckedOutToOtherUser Instructions: Pass this into PSClientError constructor to access all error information

This is the code

            //loading project data from server
            //Every change on this dataset will be updated on the server!
            ProjectDataSet projectDs = projectClient.ReadProject(projectId, projectSvc.DataStoreEnum.WorkingStore);

            foreach (projectSvc.ProjectDataSet.ProjectRow row in projectDs.Project)
            {
                if (row.PROJ_SESSION_UID != null)
                {
                    sessionId = row.PROJ_SESSION_UID;
                    break;
                }
            }
           //send the dataset to the server to update the database
           bool validateOnly = false;
           Guid jobId = Guid.NewGuid();
           projectClient.QueueUpdateProject(jobId, sessionId, projectDs, validateOnly);

Unlike other answers where we are running the code when the project is in checked-in state, we are checking-out and assigning new SessionID.
But when the event handler fires, the project is already is checked-out. So how do I get the SessionID?

I think that is where the code is breaking.

Any help will be appreciated.