protected static void isNullOperator(Operator<?> operator) throws SemanticException {
    if (operator == null) {
      throw new SemanticException("Operator is null.");
    }
  }
Throws an exception if the input operator is null

@param operator
@throws SemanticException if the input operator is null
ql/src/java/org/apache/hadoop/hive/ql/optimizer/correlation/CorrelationUtilities.java