Drools java.lang.NullPointerException at org.drools.reteoo.RuleTerminalNode

My application has to monitor when a magnetic contact is activated and when it is contact released event has to be retracted. And viceversa. Here is my rule and the big null pointer I have. Any helps will be very appreciated! Thank you a lot!!

rule "contact activated vs contact released" salience 0 no-loop true
when
$contact_activated : Event(type == EventType.CONTACT_ACTIVATED) ||

$contact_released : Event(type == EventType.CONTACT_RELEASED)
then

if($contact_activated!= null)

{ retract($contact_activated); }

else

{ retract($contact_released); }

end

and
I have

java.lang.NullPointerException
at org.drools.reteoo.RuleTerminalNode$SortDeclarations.compare(RuleTerminalNode.java:477)
at org.drools.reteoo.RuleTerminalNode$SortDeclarations.compare(RuleTerminalNode.java:473)
at java.util.Arrays.mergeSort(Arrays.java:1270)
at java.util.Arrays.sort(Arrays.java:1210)
at org.drools.reteoo.RuleTerminalNode.<init>(RuleTerminalNode.java:119)
at org.drools.RuleActivationListenerFactory.createActivationListener(RuleActivationListenerFactory.java:21)
at org.drools.reteoo.builder.ReteooRuleBuilder.addSubRule(ReteooRuleBuilder.java:157)
at org.drools.reteoo.builder.ReteooRuleBuilder.addRule(ReteooRuleBuilder.java:123)
at org.drools.reteoo.ReteooBuilder.addRule(ReteooBuilder.java:110)
at org.drools.reteoo.ReteooRuleBase.addRule(ReteooRuleBase.java:441)
at org.drools.common.AbstractRuleBase.addRule(AbstractRuleBase.java:821)
at org.drools.common.AbstractRuleBase.addPackages(AbstractRuleBase.java:555)
at org.drools.reteoo.ReteooRuleBase.addPackages(ReteooRuleBase.java:458)
at org.drools.impl.KnowledgeBaseImpl.addKnowledgePackages(KnowledgeBaseImpl.java:150)
at it.ipiu.pch.sel.RuleTest.before(RuleTest.java:1272)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:27)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:49)
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)