TypeError: Can ' T convert 'NoneType' object to str implizit

Ich bin neu in Python, also glaube ich nicht verstehen, warum ich erhalte die folgende Fehlermeldung: TypeError: Can ' T convert 'NoneType' object to str implizit

import math, os
try:
    while True:
        loadfile = input ("Get a string from input or file: ").lower()
        if loadfile == "":
            print("You need to enter whether you want the string from input or a file. Please try again")
        elif loadfile[0] != "i" and loadfile[0] != "s" and loadfile[0] != "f":
            print("You can only choose to get the string from input or a file. Please try again")
        else:
            loadfile = loadfile[0] == "f"
            raise Execption
except Exception:
    pass

if loadfile:
    while True:
        filename = input ("Enter a file name containing a string: ")
        if filename == "":
            print("You need to enter a file name, you can not leave it blank. Please try again")
        elif not os.path.isfile(filename):
            print ("This file does not exist. Please try again")
        else:
            try:
                file = open(filename, "r")
            except OSError:
                print("Your operating system threw an error")
                print("The file may be in use, or your filename may be invalid")
            else:
                string = file.read()
                file.close()
                if os.path.getsize(filename) == 0:
                        print("Found the file vut the file is empty. Enter another file")
                else:
                    flag = False
                    for x in string:
                        if x.isalpha():
                            flag = True
                    if flag:
                        break
                    else:
                        print("The string inside this file does not contain a character from the alphabet. Please try again")
    if (len(string) <= 200):
        print ("Found the string: " + string)
    else:
        print("String found, but not being printed since it it greater than 200 characters")
else:
    try:
        while True:
            string = input ("Enter your string: ");
            if string == "":
                print ("Invalid response. You need to enter a plaintext message to be able to continue")
            else:
                for x in string:
                    if x.isalpha():
                        raise Exception;
                print ("You did not enter a character from the alphabet in your plaintext message. Please try again");
    except Exception:
        pass

string2 = ""
string3 = ""

try:
    while True:
        keyword = input ("Enter your KEYWORD: ");
        if keyword == "":
            print ("You need to enter a keyword. Please try again");
        elif keyword.isalpha():
            raise Exception;
        else:
            print ("Your keyword cannot contain any numbers, spaces, or symbols. Please try again");
except Exception:
    pass

keywordRepeated = (keyword * math.ceil(len(string)/len(keyword))).upper()

try:
    while True:
        keyword2 = input ("Enter your second KEYWORD: ");
        if keyword2 == "":
            print ("You need to enter a keyword. Please try again");
        elif keyword2.isalpha():
            raise Exception;
        else:
            print ("Your keyword cannot contain any numbers, spaces, or symbols. Please try again");
except Exception:
    pass

keyword2Repeated = (keyword2 * math.ceil(len(string)/len(keyword2))).upper()        

try:
    while True:
        decryption = input("Would you like to encrypt or decrypt?: ").lower()
        if decryption == "":
            print("You need to enter whether you want to undergo the encryption or decryption process. Please try again")
        elif decryption[0] != "d" and decryption[0] != "e":
            print("You can only choose encryption or decryption. Please try again")
        else:
            decryption = decryption[0] == "d"
            raise Exception
except Exception:
    pass

try:
    while True:
        savefile = input("Should the string be saved ot the file, or displayed on screen?: ").lower()
        if savefile == "":
            print("You need to enter whether to save the string to a file or display it on screen")
        elif savefile[0] != "d" and savefile[0] != "s" and savefile[0] != "f":
            print("You can only choose to save the string to a file or display it on screen")
        else:
            savefile = savefile[0] != "d"
            raise Exception
except Exception:
    pass

firstuppercase = ord("A"); firstlowercase = ord("a"); lastuppercase = ord("Z"); lastlowercase = ord("z");

def encryptedChar(position, parString, parKeyword):
    if not parString[position].isalpha():
        return parString[position]
    charnum = ord(parString[position])
    modifier = ord(parKeyword[position].upper()) + 1 - firstuppercase
    modifier *= -1 if decryption else 1
    charnum2 = charnum + modifier

    if decryption:
        if charnum <= lastuppercase and charnum2 < firstuppercase:
            charnum += 26
        elif charnum >= firstlowercase and charnum2 < fisrtlowercase:
            charnum += 26
    else:
        if charnum <= lastuppercase and charnum2 > lastuppercase:
            charnum2 -= 26
        elif charnum >= firstlowercase and charnum2 > lastlowercase:
            charnum2 -= 26
for x in range(len(string)):
    string2 += encryptedChar(x, string, keywordRepeated)
    string3 += encryptedChar(x, string2, keyword2Repeated)

if savefile:
    while True:
        try:
            while True:
                filename = input ("Enter a filename to save to: ")
                if filename == "":
                    print("You need to enter a file name, you cannot leave it blank")
                else:
                    raise Exception
        except Exception:
            pass

        try:
            file = open(filename, "w")
            raise Exception
        except OSError:
            print("Your operating system threw an error")
            print("The file may be in use, or your file may be invalid")
        except Exception:
            file.write(string3)
            file.close()
            print("Save successful!")
            break
else:
    print(string3)

ich möchte den code ausführen, wie es unten ist, aber ich bekomme immer diese typeerror

Get a string from input or file: i
Enter your string: letters
Enter your KEYWORD: working
Enter your second KEYWORD: code
Would you like to encrypt or decrypt?: e
Should the string be saved ot the file, or displayed on screen?: d
Traceback (most recent call last):
  File "C:\Users\User\Documents\task 3.py", line 138, in <module>
    string2 += encryptedChar(x, string, keywordRepeated)
TypeError: Can't convert 'NoneType' object to str implicitly
>>>