Error: NoneType object has no attribute "used" #1423

modeller = app.Modeller(pdb_object.topology, pdb_object.positions)
force_field = openmm.app.ForceField(*force_fields)
modeller.addHydrogens(forcefield=force_field, pH=7)
modeller.addSolvent(force_field,
                                    padding = boxPadding*unit.nanometer,
                                    boxShape = "cube",
                                    ionicStrength = 0.15*unit.molar)
system = force_field.createSystem(modeller.topology,
                                          nonbondedMethod = PME,
                                          nonbondedCutoff = 1.2*ou.nanometer,
                                          constraints = HBonds)
   
restraint = openmm.CustomExternalForce('k*periodicdistance(x, y, z, x0, y0, z0)^2')
system.addForce(restraint)
restraint.addGlobalParameter('k', restraint_const*openmm.unit.kilojoules_per_mole/openmm.unit.nanometer**2)
restraint.addPerParticleParameter('x0')
restraint.addPerParticleParameter('y0')
restraint.addPerParticleParameter('z0')
...
topology_file = parmed.openmm.load_topology(modeller.topology, system)
for ff in force_fields:
             if "amber" in ff:
                 topology_file.save(f"{output_dir}/topology.prmtop")
             if "charmm" in ff:
                 topology_file.save(f"{output_dir}/topology.psf")