If trying to get thread diff from a GET request, send a message instead of failing with exception

boards/views/api.py

@@ -50,8 +50,12 b' def api_get_threaddiff(request):'
     """
 
     thread_id = request.POST.get(PARAMETER_THREAD)
-    uids_str = request.POST.get(PARAMETER_UIDS).strip()
-    uids = uids_str.split(' ')
+    uids_str = request.POST.get(PARAMETER_UIDS)
+
+    if not thread_id or not uids_str:
+        return HttpResponse(content='Invalid request.')
+
+    uids = uids_str.strip().split(' ')
 
     opening_post = get_object_or_404(Post, id=thread_id)
     thread = opening_post.get_thread()