check null

src/Tools/ResponseStrategies/TransformerTagsStrategy.php

@@ -46,7 +46,11 @@ class TransformerTagsStrategy
             $modelInstance = $this->instantiateTransformerModel($model);
 
             $fractal = new Manager();
-            $fractal->setSerializer(app(config('apidoc.fractal.serializer')));
+            
+            if(!is_null(config('apidoc.fractal.serializer'))) {
+                $fractal->setSerializer(app(config('apidoc.fractal.serializer')));
+            }
+            
             $resource = (strtolower($transformerTag->getName()) == 'transformercollection')
                 ? new Collection([$modelInstance, $modelInstance], new $transformer)
                 : new Item($modelInstance, new $transformer);