yii extensions - Error in alex-bond/yii2-thumbler "Class 'Thumbler' not found" -

how use widget alex-bond/yii2-thumbler link. tried

$path = \yii::$app->thumbler->resize         ('49_long.jpg',500,500, $method = thumbler::method_crop_center, $backgroundcolor = 'ffffff', $callexceptiononerror = true); 

as said there, got error "class 'thumbler' not found". how can use it?

try adding add correct reference code

 use alexbond\thumbler\thumbler;