Check whether ->state is null before checking its user id

src/AddTagSubscriptionAttribute.php

@@ -38,7 +38,10 @@ public function __invoke(TagSerializer $serializer, Tag $tag, array $attributes)
      */
     public function getStateFor(Tag $tag, User $actor): TagState
     {
-        if (!$tag->relationLoaded('state') || $tag->state->user_id !== $actor->id) {
+        // If $tag->state is loaded *and* null, this might be because the actor doesn't have tag-specific state
+        // OR because the wrong actor has been used for loading it. `$tag->stateFor()` will return the correct state.
+        // If it doesn't exist, it returns a dummy state with the correct actor & tag IDs.
+        if (!$tag->relationLoaded('state') || is_null($tag->state) || $tag->state->user_id !== $actor->id) {
             $tag->setRelation('state', $tag->stateFor($actor));
         }