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java.lang.NullPointerException: 	at org.drools.core.rule.Declaration.getValue(Declaration.java:228)	at com.sample.Rule_rule11017979978AccumulateExpression0Invoker.evaluate(Rule_rule11017979978AccumulateExpression0Invoker.java:19)	at org.drools.core.base.accumulators.JavaAccumulatorFunctionExecutor.accumulate(JavaAccumulatorFunctionExecutor.java:109)	at org.drools.core.rule.SingleAccumulate.accumulate(SingleAccumulate.java:82)	at org.drools.core.phreak.PhreakAccumulateNode.addMatch(PhreakAccumulateNode.java:759)	at org.drools.core.phreak.PhreakAccumulateNode.doLeftInserts(PhreakAccumulateNode.java:163)	at org.drools.core.phreak.PhreakAccumulateNode.doNode(PhreakAccumulateNode.java:80)	at org.drools.core.phreak.RuleNetworkEvaluator.switchOnDoBetaNode(RuleNetworkEvaluator.java:562)	at org.drools.core.phreak.RuleNetworkEvaluator.evalBetaNode(RuleNetworkEvaluator.java:533)	at org.drools.core.phreak.RuleNetworkEvaluator.innerEval(RuleNetworkEvaluator.java:334)	at org.drools.core.phreak.RuleNetworkEvaluator.outerEval(RuleNetworkEvaluator.java:161)	at org.drools.core.phreak.RuleNetworkEvaluator.evaluateNetwork(RuleNetworkEvaluator.java:116)	at org.drools.core.phreak.RuleExecutor.reEvaluateNetwork(RuleExecutor.java:235)	at org.drools.core.phreak.RuleExecutor.evaluateNetworkAndFire(RuleExecutor.java:106)	at org.drools.core.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1016)	at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1302)	at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1289)	at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1262)	at com.sample.DroolsTest1.main(DroolsTest1.java:20)