better contract for BootstrapWidgetTrait

BootstrapWidgetTrait.php

@@ -98,4 +98,10 @@ trait BootstrapWidgetTrait
             $this->getView()->registerJs(implode("\n", $js));
         }
     }
-}
+
+    /**
+     * @return \yii\web\View the view object that can be used to render views or view files.
+     * @see yii\base\Widget::getView()
+     */
+    abstract function getView();
+}