Method evaluate

Default version assumes that every individual is a GEIndividual. The underlying problem.evaluate() must be prepared for the possibility that some GPIndividuals handed it are in fact null, meaning that they couldn't be extracted from the GEIndividual string. You should assign

(final EvolutionState state,
        final Individual[] ind,  // the individuals to evaluate together
        final boolean[] updateFitness,  // should this individuals' fitness be updated?
        final boolean countVictoriesOnly, // don't bother updating Fitness with socres, just victories
        final int[] subpops,
        final int threadnum)