How to fix exception in Tkinter callback error?

Question

The main part of this assignment was to create a calculator but every time I push a button it pops up an error message at the top saying Exception in Tkinter callback.

This is what the entire error says:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\willi\AppData\Local\Programs\Python\Python310\lib\tkinter\__init__.py", line 1921, in __call__
    return self.func(*args)
  File "C:\Users\willi\Downloads\Reckoner.py", line 63, in <lambda>
    button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("9"))
AttributeError: 'MainGUI' object has no attribute 'process'

This is my code:

from tkinter import *
import time

#the main GUI
class MainGUI(Frame):
    def __init__(self, parent):
         Frame.__init__(self, parent, bg="white")
         parent.attributes("-fullscreen", False)
         self.setupGUI()

    def setupGUI(self):
        self.display = Label(self, text="", anchor=E, bg="white", height=2, font=("TexGyreAdventer", 50))
        self.display.grid(row=0, column=0, columnspan=4, sticky=E+W+N+S)


#configure the rows and colums of the Frame to adjust to the window
        for row in range(6):
            Grid.rowconfigure(self, row, weight=1)
        for col in range(4):
            Grid.columnconfigure(self, col, weight=1)

#the first row
# (

        img = PhotoImage(file="images/lpr.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("("))
        button.image = img
        button.grid(row=1, column=0, sticky=N+S+E+W)
# )
        img = PhotoImage(file="images/rpr.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process(")"))
        button.image = img
        button.grid(row=1, column=1, sticky=N+S+E+W)
# AC
        img = PhotoImage(file="images/clr.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("AC"))
        button.image = img
        button.grid(row=1, column=2, sticky=N+S+E+W)
# **
        img = PhotoImage(file="images/pow.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("**"))
        button.image = img
        button.grid(row=6, column=2, sticky=N+S+E+W)
# the second row
# 7
        img = PhotoImage(file="images/7.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("7"))
        button.image = img
        button.grid(row=2, column=0, sticky=N+S+E+W)
# 8
        img = PhotoImage(file="images/8.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("8"))
        button.image = img
        button.grid(row=2, column=1, sticky=N+S+E+W)

# 9      
        img = PhotoImage(file="images/9.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("9"))
        button.image = img
        button.grid(row=2, column=2, sticky=N+S+E+W)
# /
        img = PhotoImage(file="images/div.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("/"))
        button.image = img
        button.grid(row=2, column=3, sticky=N+S+E+W)
# the third row
# 4
        img = PhotoImage(file="images/4.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("4"))
        button.image = img
        button.grid(row=3, column=0, sticky=N+S+E+W)
# 5
        img = PhotoImage(file="images/5.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("5"))
        button.image = img
        button.grid(row=3, column=1, sticky=N+S+E+W)
# 6
        img = PhotoImage(file="images/6.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("6"))
        button.image = img
        button.grid(row=3, column=2, sticky=N+S+E+W)
# *
        img = PhotoImage(file="images/mul.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("*"))
        button.image = img

        button.grid(row=3, column=3, sticky=N+S+E+W)
# the fourth row
# 1
        img = PhotoImage(file="images/1.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("1"))
        button.image = img
        button.grid(row=4, column=0, sticky=N+S+E+W)
# 2
        img = PhotoImage(file="images/2.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("2"))
        button.image = img
        button.grid(row=4, column=1, sticky=N+S+E+W)
# 3
        img = PhotoImage(file="images/3.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("3"))
        button.image = img
        button.grid(row=4, column=2, sticky=N+S+E+W)
# -
        img = PhotoImage(file="images/sub.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("-"))
        button.image = img
        button.grid(row=4, column=3, sticky=N+S+E+W)
# the fifth row
# 0
        img = PhotoImage(file="images/0.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("0"))
        button.image = img
        button.grid(row=5, column=0, sticky=N+S+E+W)
# .
        img = PhotoImage(file="images/dot.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("."))
        button.image = img
        button.grid(row=5, column=1, sticky=N+S+E+W)
# =
        img = PhotoImage(file="images/eql.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("="))
        button.image = img
        button.grid(row=6, column=0, columnspan=2, sticky=N+S+E+W)
# +
        img = PhotoImage(file="images/add.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("+"))
        button.image = img
        button.grid(row=5, column=3, sticky=N+S+E+W)
# %
        img = PhotoImage(file="images/mod.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("+"))
        button.image = img
        button.grid(row=6, column=3, sticky=N+S+E+W)
        
#bak
        
        img = PhotoImage(file="images/bak.gif")
        button = Button(self, bg="white", image=img, borderwidth=0, highlightthickness=0, activebackground="white", command=lambda: self.process("<--"))
        button.image = img
        button.grid(row=1, column=3, sticky=N+S+E+W)
        
#pack the GUI
        self.pack(fill=BOTH, expand=1)
        
#processes button presses
def process(self, button):
    if (button == "AC"):
        self.display["text"] = ""
    elif (button == "="):
        expr = self.display["text"]
        try:
            result = eval(expr)
            self.display["text"] = str(result)
        except:
            self.display["text"] = "ERROR"
            time.sleep(2)
            self.display["text"] = ""
    else:
        self.display["text"] += button

##############################
# the main part of the program
##############################
# create the window
window = Tk()

# set the window title
window.title("The Reckoner")

# generate the GUI
p = MainGUI(window)

# display the GUI and wait for user interaction
window.mainloop()