this following code gives me an error leaving the task-node tn2. I don't understand what's wrong. I've checked and rechecked the code many times, and can't see what is the error.

import org.jbpm.graph.def.ProcessDefinition;
import org.jbpm.graph.exe.ProcessInstance;
import org.jbpm.graph.exe.Token;
import org.jbpm.taskmgmt.exe.TaskInstance;

public class TaskAssignmentTest {

 public void testTaskAssignment() {
 ProcessDefinition processDefinition = ProcessDefinition.parseXmlString(
 "<process-definition name='the baby process'>" +
 " <start-state>" +
 " <transition name='to_tn1' to='tn1' />" +
 " </start-state>" +

 " <task-node name='tn1'>" +
 " <task name='t1'>" +
 " </task>" +
 " <transition name='to_state1' to='state1' />" +
 " </task-node>" +

 " <state name='state1'>" +
 " <transition name='to_tn2' to='tn2' />" +
 " </state>" +

 " <task-node name='tn2'>" +
 " <task name='t2'>" +
 " </task>" +
 " <transition name='to_tn3' to='tn3' />" +
 " </task-node>" +

 " <task-node name='tn3'>" +
 " <task name='t3'>" +
 " </task>" +
 " <transition name='to_end' to='end' />" +
 " </task-node>" +

 " <end-state name='end' />" +
 "</process-definition>"
 );

 System.out.println(processDefinition.toString());

 ProcessInstance processInstance = new ProcessInstance(processDefinition);
 Token token = processInstance.getRootToken();


 token.signal();

 TaskInstance taskInstance = null;

 //TN1
 taskInstance = (TaskInstance)processInstance.getTaskMgmtInstance().getTaskInstances().iterator().next();
 System.out.println("---> " + token.getNode().getName());
 taskInstance.end("to_state1");
 //taskInstance.end();

 //STATE1
 System.out.println(">>---> " + token.getNode().getName());
 token.signal();

 //TN2
 taskInstance = (TaskInstance)processInstance.getTaskMgmtInstance().getTaskInstances().iterator().next();
 System.out.println("---> " + token.getNode().getName());
 taskInstance.end("to_tn3");

 //TN3
 taskInstance = (TaskInstance)processInstance.getTaskMgmtInstance().getTaskInstances().iterator().next();
 System.out.println("---> " + token.getNode().getName());

 processInstance.end();
 }
 public static void main(String[] args){
 System.out.println("MAIN");
 TaskAssignmentTest tat = new TaskAssignmentTest();
 tat.testTaskAssignment();
 }
}

MAIN
ProcessDefinition(the baby process)
---> tn1
>>---> state1
---> tn2
log4j:WARN No appenders could be found for logger (org.jbpm.jpdl.xml.JpdlXmlReader).
log4j:WARN Please initialize the log4j system properly.
java.lang.NullPointerException: task node does not have leaving transition 'to_tn3'
 at org.jbpm.taskmgmt.exe.TaskInstance.end(TaskInstance.java:290)
 at com.sample.jbpm.taskmgmt.TaskAssignmentTest.testTaskAssignment(TaskAssignmentTest.java:68)
 at com.sample.jbpm.taskmgmt.TaskAssignmentTest.main(TaskAssignmentTest.java:79)
Exception in thread "main"