Fix nil error in focal_point? (#6537)

app/serializers/activitypub/image_serializer.rb

@@ -19,7 +19,7 @@ class ActivityPub::ImageSerializer < ActiveModel::Serializer
   end
 
   def focal_point?
-    object.respond_to?(:meta) && object.meta['focus'].is_a?(Hash)
+    object.respond_to?(:meta) && object.meta.is_a?(Hash) && object.meta['focus'].is_a?(Hash)
   end
 
   def focal_point