Error Exception in Tkinter callback

    def change_font(self):

        def set_font():

            if self.size_check.get() == 1:
                if self.font_check.get() == 1:
                    font = ('times', 10, 'normal')
                elif self.font_check.get() == 2:
                    font = ('system', 10, 'normal')
                elif self.font_check.get() == 3:
                    font = ('courier', 10, 'normal')
                else:
                   font = ('helvetica', 10, 'normal')

            elif self.size_check.get() == 2:
                if self.font_check.get() == 1:
                    font = ('times', 12, 'normal')
                elif self.font_check.get() == 2:
                    font = ('system', 12, 'normal')
                elif self.font_check.get() == 3:
                    font = ('courier', 12, 'normal')
                else:
                    font = ('helvetica', 12, 'normal')

            elif self.size_check.get() == 3:
                if self.font_check.get() == 1:
                    font = ('times', 14, 'normal')
                elif self.font_check.get() == 2:
                    font = ('system', 14, 'normal')
                elif self.font_check.get() == 3:
                    font = ('courier', 14, 'normal')
                else:
                    font = ('helvetica', 14, 'normal')

            else:
                if self.font_check.get() == 1:
                    font = ('times', 18, 'normal')
                elif self.font_check.get() == 2:
                    font = ('system', 18, 'normal')
                elif self.font_check.get() == 3:
                    font = ('courier', 18, 'normal')
                else:
                    font = ('helvetica', 18, 'normal')

            self.window.config(font=font)
                    
            top.destroy()

        top = Toplevel(width=100, height=100)
        top.title("Cambio de Fuente")
        top.resizable(height=0, width=0)
        top.focus_set()

        self.size_check = IntVar()
        self.size_check.set(2)

        self.font_check = IntVar()
        self.font_check.set(1)

        self.style_check = IntVar()
        self.style_check.set(1)

        size_1 = Radiobutton(top, text="10", variable=self.size_check, value=1)
        size_2 = Radiobutton(top, text="12", variable=self.size_check, value=2)
        size_3 = Radiobutton(top, text="14", variable=self.size_check, value=3)
        size_4 = Radiobutton(top, text="18", variable=self.size_check, value=4)

        font_1 = Radiobutton(top, text="Times", variable=self.font_check, value=1)
        font_2 = Radiobutton(top, text="System", variable=self.font_check, value=2)
        font_3 = Radiobutton(top, text="Courier", variable=self.font_check, value=3)
        font_4 = Radiobutton(top, text="Helvetica", variable=self.font_check, value=4)

        size_1.grid(row=0, column=0)
        size_2.grid(row=0, column=1)
        size_3.grid(row=0, column=2)
        size_4.grid(row=0, column=3)

        font_1.grid(row=1, column=0)
        font_2.grid(row=1, column=1)
        font_3.grid(row=1, column=2)
        font_4.grid(row=1, column=3)

        cancel = Button(top, width=6, text="cancel", command=top.destroy, relief=RIDGE)
        cancel.grid(row=2, column=1)
        
        ok = Button(top, width=6, text="ok", command=set_font, relief=RIDGE)
        ok.grid(row=2, column=2)