Render inside render

I’m using yii\bootstrap\Tabs with render on every tab. It renders everything good. Now in one of those rendered files I have a yii\bootstrap\Modal in which I try to render another file but it does nothing.

The file is found, because if I try to render an unexisting file I get an exception. I also get an exception if I don’t pass the model to the view (‘model’ => $model).

This is how I’m rendering the file:

$this->render('/view-folder/_form', ['model' => $model]);

EDIT: forgot the echo /facepalm