Getting "cannot unpack non-iterable NoneType object" error when trying to run GA optimization - optimization

Question

I'm running through the ASE tutorial and am trying to run a GA optimization (https://wiki.fysik.dtu.dk/ase/tutorials/ga/ga_optimize.html). However, when I run the code, I get an unpacking error. I'm not too sure about how to approach the issue.
Code is as follows:
from random import random
from ase.io import write
from ase.optimize import BFGS
from ase.calculators.emt import EMT
from ase.ga.data import DataConnection
from ase.ga.population import Population
from ase.ga.standard_comparators import InteratomicDistanceComparator
from ase.ga.cutandsplicepairing import CutAndSplicePairing
from ase.ga.utilities import closest_distances_generator
from ase.ga.utilities import get_all_atom_types
from ase.ga.offspring_creator import OperationSelector
from ase.ga.standardmutations import MirrorMutation
from ase.ga.standardmutations import RattleMutation
from ase.ga.standardmutations import PermutationMutation
# Change the following three parameters to suit your needs
population_size = 20
mutation_probability = 0.3
n_to_test = 20
# Initialize the different components of the GA
da = DataConnection('gadb.db')
atom_numbers_to_optimize = da.get_atom_numbers_to_optimize()
n_to_optimize = len(atom_numbers_to_optimize)
slab = da.get_slab()
all_atom_types = get_all_atom_types(slab, atom_numbers_to_optimize)
blmin = closest_distances_generator(all_atom_types,
ratio_of_covalent_radii=0.7)
comp = InteratomicDistanceComparator(n_top=n_to_optimize,
pair_cor_cum_diff=0.015,
pair_cor_max=0.7,
dE=0.02,
mic=False)
pairing = CutAndSplicePairing(slab, n_to_optimize, blmin)
mutations = OperationSelector([1., 1., 1.],
[MirrorMutation(blmin, n_to_optimize),
RattleMutation(blmin, n_to_optimize),
PermutationMutation(n_to_optimize)])
# Relax all unrelaxed structures (e.g. the starting population)
while da.get_number_of_unrelaxed_candidates() > 0:
a = da.get_an_unrelaxed_candidate()
a.calc = EMT()
print('Relaxing starting candidate {0}'.format(a.info['confid']))
dyn = BFGS(a, trajectory=None, logfile=None)
dyn.run(fmax=0.05, steps=100)
a.info['key_value_pairs']['raw_score'] = -a.get_potential_energy()
da.add_relaxed_step(a)
# create the population
population = Population(data_connection=da,
population_size=population_size,
comparator=comp)
# test n_to_test new candidates
for i in range(n_to_test):
print('Now starting configuration number {0}'.format(i))
a1, a2 = population.get_two_candidates()
a3, desc = pairing.get_new_individual([a1, a2])
if a3 is None:
continue
da.add_unrelaxed_candidate(a3, description=desc)
# Check if we want to do a mutation
if random() < mutation_probability:
a3_mut, desc = mutations.get_new_individual([a3])
if a3_mut is not None:
da.add_unrelaxed_step(a3_mut, desc)
a3 = a3_mut
# Relax the new candidate
a3.calc = EMT()
dyn = BFGS(a3, trajectory=None, logfile=None)
dyn.run(fmax=0.05, steps=100)
a3.info['key_value_pairs']['raw_score'] = -a3.get_potential_energy()
da.add_relaxed_step(a3)
population.update()
write('all_candidates.traj', da.get_all_relaxed_candidates())
Really confused on what I should try to fix the issue. Can't return the a1 and a2 values either.