将摩尔斯电码翻译成英文而不添加额外的字典

I am making an English to Morse code and vise-versa program. I successfully made it translate English to Morse code but am having a trouble translating Morse code to English. Whenever I try to translate Morse code to English, it gives me a TypeError: “text = ''.join(map(trans_back.get, user_input)) TypeError: sequence item 0: expected str instance, NoneType found”.I am making an English to Morse code and vise-v