rfid reading function returning null
It could be your JSON object capacity is too small because of string duplication when you deserialise it to print it.
You should try a capacity like JSON_OBJECT_SIZE(3) + 500 for deserialising, and see what happens.
Also, try: doc[&quot;rfid&quot;] = str_rfid.c_str(); to see if that works better (CString instead of String object).
More information on object capacity can be found here; there's a calculator here.