Error saving ForeingKey with Post_save in User profile

I am creating the user profile with post_save signal , but I have a field foreign key that It's not being saved, and I do not see any way to solve it. I did it like this: All this from the django admin.

  

The error is: 1048, "Column 'company_id' can not be null")

MODELS.PY

#!
# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models
from django.contrib.auth.models import User
from django.core.urlresolvers import reverse

#from django.db.models import signals
from django.db.models.signals import post_save
from django.dispatch import receiver


class Empresa(models.Model):
    nombre = models.CharField(max_length=255)
    descripcion = models.TextField(max_length=200)
    slug = models.SlugField()
    created = models.DateTimeField(auto_now_add=True, editable=False)
    last_updated = models.DateTimeField(auto_now=True, editable=False)

    class Meta:
        ordering = ('-created',)

    def __unicode__(self):
        return u'%s' % self.nombre



TIPO_USUARIO = (
    (0, 'Administrador'),
    (1, 'Tecnico'),
    (2, 'Finanzas'),
    (3, 'Cliente'),
)


class PerfilUsuario(models.Model):  
    #DEFAULT_PK=1
    user = models.OneToOneField(User)#si es un cliente el nombre sera EJ: cliente@empresa
    empresa = models.ForeignKey(Empresa, related_name='empresa_administradores')
    #cuando se registren por default entren al grupo Admin, 
    tipo_usuario = models.PositiveSmallIntegerField(choices=TIPO_USUARIO, verbose_name="¿Que tipo de Usuario es?", default=0) 


    def __unicode__(self):
        return u'Perfil de: %s' % self.user.username


def create_user_profile(sender, instance, created, **kwargs):
    if created:
        PerfilUsuario.objects.get_or_create(user=instance)
        print "ssssssss"
        print instance.empresa
        PerfilUsuario.empresa = instance.empresa
        PerfilUsuario.save()
post_save.connect(create_user_profile, sender=User)



class Cliente(models.Model):

    nombre_rb = models.CharField(max_length=70, verbose_name="Nombre de usuario en RB(queue, hotspot, PPPoE)")
    ip = models.GenericIPAddressField()
    slug = models.SlugField()
    created = models.DateTimeField(auto_now_add=True, editable=False)
    last_updated = models.DateTimeField(auto_now=True, editable=False)
    # Relationship Fields
    perfil = models.OneToOneField(PerfilUsuario, )

    class Meta:
        ordering = ('-created',)

    def __unicode__(self):
        return u'%s' % self.nombre_rb

ADMIN.Py

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from django.contrib.auth.models import User
from .models import *
# Register your models here.

#Agrego el perfil del usuario al admin
class PerfilInline(admin.StackedInline):
    model = PerfilUsuario
    can_delete = False
    verbose_name_plural = 'Perfil'

#Defino el nuevo UserAdmin
class UserAdmin(UserAdmin):
    inlines = (PerfilInline, )

# Se vuelve a registar UserAdmin
admin.site.unregister(User)
admin.site.register(User, UserAdmin)

class EmpresaAdmin(admin.ModelAdmin):
    model=Empresa
    prepopulated_fields = {"slug": ("nombre",)}

admin.site.register(Empresa, EmpresaAdmin)


class ClienteAdmin(admin.ModelAdmin):
    model=Cliente
    prepopulated_fields = {"slug": ("nombre_rb",)}

admin.site.register(Cliente, ClienteAdmin)

Please can you tell me, what am I doing wrong in post_save ? Thank you very much.

SOLUTION: It was not necessary to run the Post_save signal, since it was working from the admin. When I have to create a form in my template, I will check if this problem happens. So far it works. Thanks @deoxyseia