NullPointerException in ClassFieldReader when trying to deserialise knowledgebase

Issue

• When we deserialize a knowledgebase and try to run rules, we get a NPE.

java.lang.NullPointerException
  at org.drools.base.ClassFieldReader.getValue(ClassFieldReader.java:87)
  at org.drools.base.evaluators.EqualityEvaluatorsDefinition$StringEqualEvaluator.evaluate(EqualityEvaluatorsDefinition.java:1995)
  at org.drools.rule.LiteralRestriction.isAllowed(LiteralRestriction.java:87)
  at org.drools.rule.LiteralConstraint.isAllowed(LiteralConstraint.java:109)
  at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:131)
  at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:458)
  at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:376)
  at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:214)
  at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:244)
  at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:337)
  at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:298)
  at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:887)
  at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:846)
  at org.drools.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:267)