NullPointerException when ExecutionImpl.createVariable() with isHistoryEnabled=true

When I want to create a history enabled variable, the ProcessInstanceImpl will invoke variable.seValue(), in this method, variable will fire a VariableUpdate command. This command will try to get the HistoryVariableImpl and update it. But at this moment, the HistoryVariableImpl has not been created, so there will be a NullPointerException.

I think we should check whether the HistoryVariableImpl instance exists, then decide to update it or not.

The used code is like this:
ProcessInstance processInstance = executionService.startProcessInstanceByKey("Normal");

EnvironmentImpl environment = ((EnvironmentFactory)processEngine).openEnvironment();
((ExecutionImpl)processInstance).createVariable("test", "valuetest","string",true);
Session session = environment.get(Session.class);
Transaction tx = session.beginTransaction();
session.update(processInstance);
tx.commit();
environment.close();

The exception stack trace is like this:

15:27:28,703 SEV | [BaseJbpmTestCase] TEST THROWS EXCEPTION: null
java.lang.NullPointerException
at org.jbpm.pvm.internal.history.events.VariableUpdate.process(VariableUpdate.java:48)
at org.jbpm.pvm.internal.history.HistorySessionImpl.process(HistorySessionImpl.java:31)
at org.jbpm.pvm.internal.history.HistoryEvent.fire(HistoryEvent.java:62)
at org.jbpm.pvm.internal.history.HistoryEvent.fire(HistoryEvent.java:52)
at org.jbpm.pvm.internal.type.Variable.setValue(Variable.java:97)
at org.jbpm.pvm.internal.model.ScopeInstanceImpl.createVariableObject(ScopeInstanceImpl.java:140)
at org.jbpm.pvm.internal.model.ScopeInstanceImpl.createVariable(ScopeInstanceImpl.java:90)
at com.asset.jupiter.jbpm.processes.Normal.NormalTest.testCreateProcessInstance(NormalTest.java:33)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at junit.framework.TestCase.runTest(TestCase.java:164)
at org.jbpm.test.BaseJbpmTestCase.runTest(BaseJbpmTestCase.java:80)
at junit.framework.TestCase.runBare(TestCase.java:130)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:120)
at org.eclipse.jdt.internal.junit.runner.junit3.JUnit3TestReference.run(JUnit3TestReference.java:130)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)