fix compiler warning

panda/src/tform/mouseInterfaceNode.cxx

@@ -140,7 +140,7 @@ check_button_events(const DataNodeTransmit &input,
   const ButtonEventList *button_events = NULL;
 
   if (input.has_data(_button_events_input)) {
-    DCAST_INTO_R(button_events, input.get_data(_button_events_input).get_ptr(), false);
+    DCAST_INTO_R(button_events, input.get_data(_button_events_input).get_ptr(), NULL);
     button_events->update_mods(_current_button_state);
   }