src/widget/Button.php
public function init()
    {
        parent::init();
        $this->clientOptions = false;
        Html::addCssClass($this->options, 'btn');
    }
public function init()
    {
        parent::init();
        $this->clientOptions = false;
        Html::addCssClass($this->options, 'btn');
    }
Initializes the widget.
If you override this method, make sure you call the parent implementation first.