NameError: name 'ntxtbox' is not defined, even though ntxtbox is a global variable

Question

In the below program I have 2 text widgets. The names for these widgets have been made into global variables so I can access them in the function 'clickedCipher' The first one, inserttext, is recognized, but when I get to ntxtbox I get the following error.

Traceback (most recent call last):
File "C:/Users/sethb/PycharmProjects/TextCipher/Cipher 2.0.py", line 122, in <module>
app = tkinterApp()
File "C:/Users/sethb/PycharmProjects/TextCipher/Cipher 2.0.py", line 16, in __init__
frame = F(container, self)
File "C:/Users/sethb/PycharmProjects/TextCipher/Cipher 2.0.py", line 52, in __init__
cipher = ttk.Button(self, text='Cipher', command= clickedCipher())
File "C:/Users/sethb/PycharmProjects/TextCipher/Cipher 2.0.py", line 119, in clickedCipher
ntxtbox.delete('sel.first', tk.END)
NameError: name 'ntxtbox' is not defined

I cannot figure out why the one Text widget shows as defined but the other doesn't. A friend tried looking at it and said it needed to be registered, but he has never used python before.

import tkinter as tk
from tkinter import messagebox
from tkinter import ttk

class tkinterApp(tk.Tk):
    def __init__(self, *args, **kwargs):
        tk.Tk.__init__(self, *args, **kwargs)

        container = tk.Frame(self)
        container.pack(side="top", fill="both", expand=True)
        container.grid_rowconfigure(0, weight=1)
        container.grid_columnconfigure(0, weight=1)

    self.frames = {}
    for F in (mainWindow, newWindow):
        frame = F(container, self)
        self.frames[F] = frame
        frame.grid(row=0, column=0, sticky="nsew")

    self.show_frame(mainWindow)

def show_frame(self, cont):
    frame = self.frames[cont]
    frame.tkraise()

class mainWindow(tk.Frame):
    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)

        lbl = ttk.Label(self,
                text="Welcome to my Caesar Cipher.  To begin the cipher press start.\nTo learn more about"
                        " the Caesar Cipher press facts.")
        lbl.grid(column=0, row=0)

        factsbtn = ttk.Button(self, text="Facts", command=clickedFacts)
        factsbtn.grid(row=2, column=1, padx=10, pady=10)

        startbtn = ttk.Button(self, text="Start", command=lambda: controller.show_frame(newWindow))
        startbtn.grid(row=1, column=1, padx=10, pady=10)

class newWindow(tk.Frame):
    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)
        global inserttxt
        enterlbl = ttk.Label(self, text="Enter your text below and click Cipher to run the cipher program")
        enterlbl.grid(row=0)

        inserttxt = tk.Text(self)
        inserttxt.insert('1.0', 'enter your message here')
        inserttxt.grid(row=1)

        cipher = ttk.Button(self, text='Cipher', command=clickedCipher())
        cipher.grid(row=2)

        outputlbl = ttk.Label(self, text="The ciphered text is:")
        outputlbl.grid(row=3)

        global ntxtbox

        ntxtbox = tk.Text(self)
        ntxtbox.insert('1.0', 'Ciphered message')
        ntxtbox.grid(row=4)

def clickedFacts():
    History='    The Caesar Cipher is an encryption method that dates back to the 1st century B.C.  It is named' \
        ' after Julias Caesar who used it to send military messages across his empire.  It is believed that when' \
        ' his enemies found it, they assumed the messages were written in another language and simply did not try' \
        ' to decipher it.\n \n    My cipher uses an 8 letter shift, meaning an A will be written as an I.  Julius' \
        ' Caesar is believed to have used a shift of 3, while his nephew used a shift of only 1'
    messagebox.showinfo('Facts', History)

def split(word):
    return [char for char in word]

def shift(indexes):

    if indexes < 25 - 8:
        indexes += 8
    else:
        indexes -= 17
    return indexes

def intoCipher(otext, p, k):
    arrLow = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
    arrCap = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']
    indexA = 0
    lowOrUp = '0'
    b = 0
    x = len(otext)
    index = 0
    while index < x:
        while indexA < 25 and lowOrUp == '0':
            if otext[index] == arrLow[indexA]:
                lowOrUp = '1'
                p = indexA
                b = 1
            elif otext[index] == arrCap[indexA] and lowOrUp == '0':
                lowOrUp = '1'
                p = indexA
                b = 2
            indexA += 1
        k = shift(p)
        if b == 1:
            otext[index] = arrLow[k]
        elif b == 2:
            otext[index] = arrCap[k]
        b = 0
        lowOrUp = '0'
        index += 1
        indexA = 0
    return otext

def clickedCipher():
    global string
    string = inserttxt.get('1.0', tk.END)
    string = split(string)
    string = intoCipher(string, 0, 0)
    string = ''.join(string)
    ntxtbox.delete('sel.first', tk.END)
    ntxtbox.insert('sel.first', string)

app = tkinterApp()
app.mainloop()