4385             int was_shared = cached == ((PyDictObject *)dict)->ma_keys;         
4386             res = PyDict_SetItem(dict, key, value);                             
4387             if (was_shared && cached != ((PyDictObject *)dict)->ma_keys) {      
4388                 /* PyDict_SetItem() may call dictresize and convert split table 
...
4401                  */                                                             
4402                 if (cached->dk_refcnt == 1) {                                   
4403                     CACHED_KEYS(tp) = make_keys_shared(dict);                   
4404                 }                                                               
4405                 else {                                                          
4406                     CACHED_KEYS(tp) = NULL;                                     
4407                 }

L4402 accessed free `cached` object.
At PyDict_SetItem() in L4386, some callback is called through weakref callback,
and the callback inserts something into this dict.  shared key object (cached) is freed.

So right way to fix it may be DK_INCREF() before PyDict_SetItem().