Validation in the Model not working

Hi all,
I am trying to perform data validation in my Model class, following along with the instruction/practices mentioned in the docs here. I am getting the error: 

PHP Code:

Call to a member function setRules() on null 


 
Here is my code:
My form has this code:

PHP Code:

<?=$this->extend("layouts/base");?>

<?=$this->section("title");?>
<?=$page_title;?>
<?=$this->endSection();?>

<?=$this->section("content");?>
<div class="container-login">
<?php if (! empty($errors)) : ?>
    <div class="alert alert-danger">
    <?php foreach ($errors as $field => $error) : ?>
        <p><?= $error ?></p>
    <?php endforeach; ?>
    </div>
<?php endif; ?>
<div class="container">
<div class="row align-items-center justify-content-center">
<?=form_open(base_url('/walks/create'), ['class' => "shadow p-3 mb-5 bg-white rounded"]);?>
<div class="form-group">
<?=form_label('Day of Walk', 'day_walked');?>
<?=form_input(['name' => 'day_walked', 'type' => 'date', 'id' => 'day_walked', 'class' => 'form-control', 'value' => set_value('day_walked', '')]);?>
<span class="invalidFeedback" id="day_walked_span">
</span>
</div>
<div class="form-group">
<?=form_label('Calories Burned', 'cal_burned');?>
<?=form_input(['name' => 'cal_burned', 'class' => 'form-control', 'type' => 'number', 'step' => '0.01', 'id' => 'cal_burned', 'value' => set_value('cal_burned', '')]);?>
<span class="invalidFeedback" id="cal_burned_span">
</span>
</div>
<div class="form-group">
<?=form_label('Distance of Walk', 'miles_walked');?>
<?=form_input(['name' => 'miles_walked', 'class' => 'form-control', 'type' => 'number', 'step' => '0.01', 'id' => 'miles_walked', 'value' => set_value('miles_walked', '')]);?>
<span class="invalidFeedback" id="miles_walked_span">
</span>
</div>
<div class="form-group">
<?=form_label('Duration of Walk', 'duration');?>
<?=form_input(['name' => 'duration', 'class' => 'form-control', 'type' => 'text', 'id' => 'duration', 'placeholder' => 'HH:MM:SS', 'value' => set_value('duration', '')]);?>
<span class="invalidFeedback" id="duration_span">
</span>
</div>
<div class="form-group">
<?=form_label('Pace', 'mph');?>
<?=form_input(['name' => 'mph', 'class' => 'form-control', 'type' => 'number', 'step' => '0.01', 'id' => 'mph', 'value' => set_value('mph', '')]);?>
<span class="invalidFeedback" id="mph_span">
</span>
</div>
<div class="form-group">
<?=form_label('Shoe Worn', 'shoe_id');?>
<?=form_dropdown('shoe_id', $shoes, ['id' => 'shoe_id']);?>
<span class="invalidFeedback" id="shoe_id_span">
</span>
</div>
<div class="row">
<div class="col offset-1">
<?=anchor(base_url('/walks'), 'Cancel', ['class' => 'btn btn-primary', 'role' => 'button']);?>
</div>
<div class="col offset-2">
<?=form_submit('walk_add_btn', 'Add Walk', ['id' => 'walk_add_btn', 'class' => 'btn btn-primary']);?>
</div>
</div>
<?=form_close();?>
</div>
</div>
</div>
<?=$this->endSection();?>

Here is the controller and method I am using to process the form data:

PHP Code:

<?php
namespace App\Controllers;

use App\Models\WalkModel;
use App\Models\ShoeModel;

class Walks extends BaseController
{

    public $walk_model;
    public $shoe_model;
    public $shoes;
    
    public function __construct() {
    $this->walk_model = new WalkModel();
    $this->shoe_model = new ShoeModel();
        $this->shoes = $this->shoe_model->findAll();
    }

    public function index() {
    $data = [
    'page_title' => 'All Walks',
    'walks' => $this->walk_model->getAllWalks(),
    'shoes' => $this->shoes,
            'errors' => NULL,
    ];

    return view('walks/w_index', $data);
    }

    public function create() {
        //Creating a $shoes array for the dropdown where they are needed
        foreach ($this->shoes as $shoe) {
            $shoes[$shoe->shoe_id] = $shoe->brand_name;
        }
        //default ordering of shoes by brand_name ASC
        array_multisort($shoes, SORT_ASC);

        $data = [
            'page_title' => 'Add A Walk',
            'shoes' => $shoes,
            'errors' => NULL
        ];

        if ($this->request->getMethod() == 'post') {
            
            $walk_data = [
                'day_walked' => $this->request->getVar('day_walked', FILTER_SANITIZE_STRING),
                'cal_burned' => $this->request->getVar('cal_burned', FILTER_SANITIZE_STRING),
                'miles_walked' => $this->request->getVar('miles_walked', FILTER_SANITIZE_STRING),
                'duration' => $this->request->getVar('duration', FILTER_SANITIZE_STRING),
                'mph' => $this->request->getVar('mph', FILTER_SANITIZE_STRING),
                'shoe_id' => $this->request->getVar('shoe_id', FILTER_SANITIZE_STRING) 
            ];

            if ($this->walk_model->insert($walk_data) === false) {
                $data['errors'] = $this->walk_model->errors();
            } else {
                return redirect()->to(base_url('walks'));
            }
            
        }// post request method ends here

        return view('walks/create', $data);
    } //create method ends here

    public function update() {
    //not implemented
    }

    public function delete() {
    //not implemented
    }
} 


And here is my Model class that I am trying to use for the validation:

PHP Code:

<?php
namespace App\Models;

use CodeIgniter\Model;

class WalkModel extends Model
{
protected $table      = 'walking_stats';
protected $primaryKey = 'day_walked';
protected $returnType    = 'object';
protected $allowedFields = ['day_walked', 'cal_burned', 'miles_walked', 'duration', 'mph', 'shoe_id'];
protected $db;
protected $builder;

protected $validationRules = [
'day_walked' => 'required|is_unique[walking_stats.day_walked]',
'cal_burned' => 'required|decimal|greater_than[0]',
'miles_walked' => 'required|decimal|greater_than[0]',
'duration' => 'required|exact_length[8]',
'mph' => 'required|decimal|greater_than[0]',
'shoe_id' => 'required|integer'
  ];

  protected $validationMessages = [
  'day_walked' => [
  'required' => 'Must choose a date value',
  'is_unique' => 'A walk has already been recorded for date {param}',
  ],
  'cal_burned' => [
  'required' => 'Calories burned value is required',
  'decimal' => 'Calories burned must be in decimal number format',
  'greater_than' => 'Calories burned value must be greater than 0 (zero)',
  ],
  'miles_walked' => [
 'required' => 'A distance walked value is required',
  'decimal' => 'The distance walked value must be in decimal number format',
  'greater_than' => 'The distance walked value must be greater than 0 (zero)',
  ],
  'duration' => [
  'required' => 'A duration value is required',
  'exact_length' => 'Format error - Not enough values. Duration value must be in HH:MM:SS format',
  ],
  'mph' => [
  'required' => 'The pace of the walk value is required',
  'decimal' => 'The pace of the walk value must be in decimal number format',
  'greater_than' => 'The pace of the walk value must be greater than 0 (zero)', 
  ],
  'shoe_id' => [
  'required' => 'Must choose from the drop-down selection of shoes',
  'integer' => 'Only choices from the drop-down selection are allowed'
  ]
  ];

  protected $skipValidation    = false;

public function __construct() {
$this->db      = \Config\Database::connect();
$this->builder = $this->db->table($this->table); 
}

public function getAllWalks() {
$this->builder->select('day_walked, cal_burned, miles_walked, duration, mph, brand_name')
->join('shoes_worn', 'walking_stats.shoe_id = shoes_worn.shoe_id');
return $this->builder->get()->getResult();
}

} 


I am new at CI4 and not really sure where I am going wrong with this.
I am able to

Code:

var_dump($walk_data)

in the controller so I am getting the values from the form without any issues.
Any help, suggestions, or ideas is most appreciated.
Thanks all,