posts not showing after updating or creating codeigniter

Question

My question is why posts are not showing after updating or creating in codeigniter? for example if I create a post without assigning a category in the select html tag it creates the post in the database but is not showing in the application and same if update the post without select a category in select html tag. But if I assign a category it shows the post.

Here is my code

Model category

public function pelicula_categoria()
{
    $this->db->order_by('categoria_nombre');
    $query = $this->db->get('categorias');
    return $query->result_array();
}

Model post

public function listar_peliculas()
{
    $this->db->select();
    $this->db->join('categorias', 'categorias.categoria_id = peliculas.categoria_id');
    $this->db->from('peliculas');
    $this->db->order_by('peliculas.pelicula_id', 'desc');

    $query = $this->db->get();
    return $query->result_array();
}

Controller add post

public function agregar_pelicula()
{
    $data['error'] = null;
    $data['titulo_agregar'] = 'Agregar pelicula';
    $data['categorias'] = $this->Categorias_Model->pelicula_categoria();
    if ($this->input->post())
    {
        $this->form_validation->set_rules('pelicula_titulo', 'Titulo', `     'required');
        $this->form_validation->set_rules('pelicula_sinopsis', 'Sinopsis', 'required');
        $this->form_validation->set_rules('pelicula_caratula', 'Caratula', 'required');
        $this->form_validation->set_rules('pelicula_estreno', 'Estreno', 'required');
        $this->form_validation->set_rules('pelicula_puntuacion', 'Puntuacion', 'required');
        $this->form_validation->set_rules('pelicula_director', 'Director', 'required');
        $this->form_validation->set_rules('pelicula_duracion', 'Duracion', 'required');
        $this->form_validation->set_rules('pelicula_pais', '', 'required');
        $this->form_validation->set_rules('categoria_id', 'Categoria', 'required');
        $this->form_validation->set_rules('pelicula_enlace', 'Enlace', 'required');

        if ($this->form_validation->run() == FALSE)
        {
            $data['error'] = "Todos los campos son requeridos";
        }
        else
        {
            $this->Peliculas_Model->insert_pelicula();
            $this->session->set_flashdata('mensaje', 'Pelicula agregada correctamente');
            redirect(base_url() . 'admin');
        }

    }
    $this->load->view('admin/layouts/header', $data);
    $this->load->view('admin/modules/create', $data);
    $this->load->view('admin/layouts/footer');
}

And finally showing the categories from database in create view and update view

<select class="custom-select" name="categoria_id">
                            <option selected>Categoria</option>
                            <?php foreach ($categorias as $categoria): ?>
                                <option value="<?php echo $categoria['categoria_id']; ?>">
                                    <?php echo $categoria['categoria_nombre']; ?>
                                </option>
                            <?php endforeach; ?>
                        </select>

Answer 1

Did you check in print_r(); function?

 //example - Model
    public function userlists()
    {
      $data = '';       
      $sql = "SELECT * FROM customer";
      $query = $this->db->query($sql);
      if($query->num_rows()>0)
       {
           return  $query->result_array(); 
       }

   } 

//Controller
        $data['userlist']= $userslists=$this->example_model->userlists();

//check the array values is exists or not, just print the variable
print_r($userslists);