attribute param should be $attribute->name, not $attribute

README.md

@@ -141,7 +141,7 @@ $activeFields = [
    'common\models\Foo.isAvailable' => function ($attribute, $generator) {
        $data = \yii\helpers\VarDumper::export([0 => 'Nein', 1 => 'Ja']);
        return <<<INPUT
-\$form->field(\$model, '{$attribute}')->checkbox({$data});
+\$form->field(\$model, '{$attribute->name}')->checkbox({$data});
 INPUT;
    },