Why does ProcessInstance throw an RuntimeException: Process instance XXX is disconnected or NPE on invocation of its method in BPMS ?

Issue

• I am starting a process that goes to wait state and will only resume upon receiving a signal. While the process instance is in wait state, I am trying is to get some info on the process instance (using getter methods like getProcessName() ) . I have observed, if I start the process instance using the following way [1] then it works fine, otherwise if I use the other approach [2] it complains with a NullPointerException as shown below [3] inside the stack trace.
  Can you please advise what is the correct approach amongst [1] & [2] and why it complains the NullPointerException ?

[1]

StatefulKnowledgeSession statefulKnowledgeSession = JPAKnowledgeService.newStatefulKnowledgeSession(kb,null,env);
ProcessInstance processInstance = knowledgeSession.createProcessInstance(processName, params);
knowledgeSession.startProcessInstance(processInstance.getId());

[2]

StatefulKnowledgeSession statefulKnowledgeSession = JPAKnowledgeService.newStatefulKnowledgeSession(kb,null,env);
ProcessInstance processInstance = knowledgeSession.createProcessInstance(processName, params);
processInstance.start();

[3]

java.lang.NullPointerException
        at org.jbpm.process.instance.impl.ProcessInstanceImpl.getProcess(ProcessInstanceImpl.java:67)
        at org.jbpm.process.instance.impl.ProcessInstanceImpl.getProcessName(ProcessInstanceImpl.java:81)
        ...

• I get NullPointerException when I call WorkflowProcessInstance.setVariable().

java.lang.NullPointerException
    at org.jbpm.process.instance.impl.ProcessInstanceImpl.getProcess(ProcessInstanceImpl.java:67)
    at org.jbpm.process.instance.impl.ProcessInstanceImpl.getContextInstance(ProcessInstanceImpl.java:127)
    at org.jbpm.workflow.instance.impl.WorkflowProcessInstanceImpl.setVariable(WorkflowProcessInstanceImpl.java:238)
    at com.sample.ProcessTest.testProcess(ProcessTest.java:60)
...

• I get RuntimeException when I call ProcessInstance.getProcess() in BPMS 6.1.

java.lang.RuntimeException: Process instance 1[com.sample.bpmn.hello] is disconnected.
    at org.jbpm.process.instance.impl.ProcessInstanceImpl.getProcess(ProcessInstanceImpl.java:100)
    at com.sample.ProcessJPATest.testProcess(ProcessJPATest.java:101)

• I get RuntimeException when I call JbpmJUnitBaseTestCase.assertNodeActive() in BPMS 6.1.

java.lang.RuntimeException: Process instance 1[com.sample.bpmn.hello] is disconnected.
        at org.jbpm.process.instance.impl.ProcessInstanceImpl.getProcess(ProcessInstanceImpl.java:100)
        at org.jbpm.workflow.instance.impl.WorkflowProcessInstanceImpl.getWorkflowProcess(WorkflowProcessInstanceImpl.java:227)
        at org.jbpm.workflow.instance.impl.WorkflowProcessInstanceImpl.getNodeContainer(WorkflowProcessInstanceImpl.java:85)
        at org.jbpm.workflow.instance.impl.NodeInstanceImpl.getNode(NodeInstanceImpl.java:121)
        at org.jbpm.workflow.instance.node.WorkItemNodeInstance.getNodeName(WorkItemNodeInstance.java:383)
        at org.jbpm.test.JbpmJUnitBaseTestCase.assertNodeActive(JbpmJUnitBaseTestCase.java:677)
        at org.jbpm.test.JbpmJUnitBaseTestCase.assertNodeActive(JbpmJUnitBaseTestCase.java:664)
        at com.sample.ProcessTest.testProcess(ProcessTest.java:30)