Yii2: Rendering view from within the widget

What have I tried?

1. I have created CustomWidget by extending Widget class

2. In the end of run method body I tried to place by different ways calling render method.

3. I have tried this ways

$this->render( '//widgets/custom' )

$this->render( 'custom' )

$this->render( '@frontend/views/widgets/custom' )

$this->renderFile(/*absolute path to file*/)

But all in vain.

And with another view, for example

$this->render('//site/404') - Works

I thought maybe the problem the widget is searching for different directory than all other views. I have overridden Widget method getViewPath and intentionally return path to my views. By default getViewPath returns .../components/views. But the error is still exists. I have tried to place widget directory to components/views, but there is no result.

Piece of CustomWidget code

class CustomWidget extends Widget {
    public function run() {
        return $this->render( '/widgets/custom' );
    }
}

I'll be grateful for any clue, suggestion, advice about fixing this problem!