trying to solve IT problems

I am in the process of merging two applications. These applications have a similar structure and similar dependencies.
However, one of the applications uses Drools 5.4.0.Final because it needs an accumulate function which does not work in earlier versions. The other application runs fine using 5.3.1.Final but fails when updating to 5.4.

Though the exception is masked by other errors inside the application, some digging revealed the following:

java.lang.NullPointerException
at java.lang.String.<init>(String.java:177)
at org.mvel2.util.ErrorUtil.rewriteIfNeeded(ErrorUtil.java:12)
at org.mvel2.compiler.ExpressionCompiler._compile(ExpressionCompiler.java:302)
at org.mvel2.compiler.ExpressionCompiler.compile(ExpressionCompiler.java:62)
at org.mvel2.MVEL.compileExpression(MVEL.java:810)
at org.drools.base.mvel.MVELCompilationUnit.compile(MVELCompilationUnit.java:442)
at org.drools.base.mvel.MVELCompilationUnit.getCompiledExpression(MVELCompilationUnit.java:238)
at org.drools.rule.constraint.MvelConstraint.createMvelConditionEvaluator(MvelConstraint.java:173)
at org.drools.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:157)
at org.drools.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:124)
at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:137)
at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:497)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:382)
at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:141)
at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:497)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:382)
at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:235)
at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:240)
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:888)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:847)
at org.drools.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:269)
at org.drools.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:303)
...

Not nice.
Some debugging shows that the problem in the following rule:

rule "Check estimate values"
    when
        kb: Calculation(
            null == totalEstimateExVat || totalEstimateExVat <= 0 ||
            null == vatAmountEstimate || vatAmountEstimate < 0 ||
            Math.abs(totalEstimateExVat + vatAmountEstimate - totalEstimateWithVat) > .00001 )
    then
        validationException.addException("Estimate values are incorrect." );
end

Apparently, something changes in the way the expressions are evaluated. While it is a good practice in Java to do an equality test with the constant on the left (it avoids accidental assignment when typing = instead of ==), it seems that this coding style is no longer valid in Drools 5.4. Reverting the tests fixed the problem.