don't test tree type outside of new thread start

misago/threads/api/postingendpoint/category.py

@@ -17,7 +17,9 @@ class CategoryMiddleware(PostingMiddleware):
     Middleware that validates category id and sets category on thread and post instances
     """
     def use_this_middleware(self):
-        return self.mode == PostingEndpoint.START and self.tree_name == THREADS_ROOT_NAME
+        if self.mode == PostingEndpoint.START:
+            return self.tree_name == THREADS_ROOT_NAME
+        return False
 
     def get_serializer(self):
         return CategorySerializer(self.user, data=self.request.data)

misago/threads/api/postingendpoint/participants.py

@@ -14,8 +14,8 @@ from ...permissions import allow_message_user
 
 class ParticipantsMiddleware(PostingMiddleware):
     def use_this_middleware(self):
-        if self.tree_name == PRIVATE_THREADS_ROOT_NAME:
-            return self.mode == PostingEndpoint.START
+        if self.mode == PostingEndpoint.START:
+            return self.tree_name == PRIVATE_THREADS_ROOT_NAME
         return False
 
     def get_serializer(self):

misago/threads/api/postingendpoint/privatethread.py

@@ -9,8 +9,8 @@ class PrivateThreadMiddleware(PostingMiddleware):
     Middleware that sets private threads category for thread and post
     """
     def use_this_middleware(self):
-        if self.tree_name == PRIVATE_THREADS_ROOT_NAME:
-            return self.mode == PostingEndpoint.START
+        if self.mode == PostingEndpoint.START:
+            return self.tree_name == PRIVATE_THREADS_ROOT_NAME
         return False
 
     def pre_save(self, serializer):