Validation div always appears in view even if empty

Hi,
I'm quite new to CodeIgniter and website development in general.
I'm following various tutorials and examples as well the official documentation, however I'm having some difficulties with displaying validation errors in my view.
Here's my controller section related to validation of a form input:

PHP Code:

public function add()
 {
 helper('form');

 $model = new ReportModel();

 if (!$this->validate([
            'modello'  => 'required|numeric',
            'matricolaStruttura' => 'alpha_numeric|max_length[10]',
            'matricolaCabina'  => 'alpha_numeric|max_length[10]',
            'matricolaPorta'  => 'alpha_numeric|max_length[10]',
            'difetto' => 'required|numeric',
            'responsabile' => 'required|numeric',
        ]))
{
 return redirect()->to('defect')->withInput()->with('validation',$this->validator);
}
else 
[...] 


and here's my view where I want to show validation errors if they happen:

PHP Code:

<?php $validation = \Config\Services::validation(); ?>
    <?php if ($validation): ?>
      <div class="alert alert-danger" role="alert">
        <?= $validation->listErrors() ?>
      </div>
    <?php endif; ?>

in this case the result is that whenever I reach the view (even if post action has not been sent) I see the "alert div" block obviously empty of content. Of course I want it to be empty in that case.
If I remove the line

PHP Code:

<?php $validation = \Config\Services::validation(); ?>

what happens is that I get CI error of undefined $validation variable. I expect that the $validation variable is passed to the view with the option, but it seems not working.

PHP Code:

withInput()->with('validation', $this->validator) 


The code I wrote was extracted from this tutorial: https://makitweb.com/crud-create-read-up...igniter-4/ however I don't want to show errors in each form cell but in a div at top of the form.

Any suggestion? What's the best practice to have the validation errors shown as I want to?
Thanks in advance