'NoneType' object has no attribute '_nserie' - Python Collections

I have the following classes:

class Contenedor:
    def __init__(self, nserie, categoria, peso):
        self._nserie = nserie
        self._categoria = categoria
        self._peso = peso

        @property
        def categoria(self):
            return self._categoria

        @property
        def peso(self):
            return self._peso

class Patio:
    def __init__(self, cantSitios, cantContenedores):
        self._sitios = [None] * cantSitios
        self._pLibreSitios = 0
        self._almacenamiento = []

        for i in range(cantSitios):
            self._almacenamiento.append([None] * cantContenedores)

A patio class, which has as attributes an arrangement of places (places), and a storage matrix that will allow me to store containers (class Container) in a certain position associated with a site

I'm trying to do the following method to add containers to the matrix

def agregarContenedor(self, cont, nSitio):
    busqSitio = self.buscarSitio(nSitio)
    if busqSitio != -1:
        for j in range(len(self._almacenamiento[busqSitio])):
            if self._almacenamiento[busqSitio][j] is None and self._almacenamiento[busqSitio][j]._nserie != cont._nserie:
                self._almacenamiento[busqSitio][j] = cont


            else:
                return -1
    else:
        return -1

The method itself is adding a container (cont) verifying that the site named nSite is found, in addition to the container being added in a position where space is available (be None). In addition you must verify that the serial number of the container to be added, is not repeated in the matrix, the latter I am trying to do with the line self._storage [siteSearch] [j] ._ nserie! = Cont._nserie but in doing so, I an error 'NoneType' object has no attribute '_nserie

Does anyone know how to fix it ?, I'm starting in Python and I do not understand where the error may be. Thanks in advance