Drools NullPointerException causes RuntimeDroolsException

Issue

• A user-supplied method caused a RuntimeDroolsException.

  Exception:
  
  (Stack trace exceprt):
  org.drools.RuntimeDroolsException: Exception executing  ReturnValue constraint org.drools.rule.ReturnValueRestriction@9cf19fa6 :  java.lang.NullPointerException
   at org.drools.rule.ReturnValueConstraint.isAllowed(ReturnValueConstraint.java:138)
  .....
  Caused by: java.lang.NullPointerException
   at java.util.Date.getMillisOf(Date.java:939)
  

• The user believed this error was caused by a simple "empty" check, because this was one of the conditions checked in the rule.  In fact, it was a secondardy condition on the rule.  The rule reads as follows:

  mybean : MyBean( someDateField1 != ( empty ) , someDateField2 < ( mybean.convertStringToDate(someTextFieldHoldingADate) ) )