Template File Location

As per code given at https://www.agiletoolkit.org/intro/1 when I am using following code in my program:

class Plane extends \atk4\ui\View
    public $defaultTemplate = './plane.html';

    protected $num = null; 

    protected $icon = null;

    function renderView() {
        if (!is_object($this->icon)) {
            $this->icon = new Icon($this->icon);

        $this->add($this->icon, 'Plane_Spot');
        $this->template->set('num', isset($this->num) ? $this->num : rand(5, 20));        
        return parent::renderView();

// default icon and random number
$plane = $app->layout->add(new Plane());

// specify icon and number
$plane = $app->layout->add(new Plane(['icon'=>'book', 'num'=> 20]));

// you can also do this:
$plane = $app->layout->add(new Plane());
$plane->icon = 'book';
$plane->num = 20;

// you can also inject your own custom (or 3rd party) icon object
$plane->icon = new MyCustomIcon('abc');

Then I am getting the following error message:

Fatal Error
atk4\ui\Exception: Unable to read template from file

Please note that I have saved the file plain.html in the folder vendor/atk4/ui/template/semantic-ui/ but even after that it is not being used by the code. Please help me what is wrong with this code?

Best Regards …
Pankaj Kumar

I thing so pug file also need for template
some info