[Tkinter] manipulation of string in Text widget

[Stauricus]

hello everybody
I'm trying to create a simple text editor in Tkinter, to help building HTML pages.
I just write things in the text area, and use buttons to add tags, like <h1>.

this is what I have so far:

from tkinter import *
root = Tk()

def addHeader():
	a = main_text.get(Tk.SEL_FIRST, Tk.SEL_LAST)


main_text = Text(root).grid(row=0, column=0, rowspan=6)

b_header = Button(root, text ="Header", command=addHeader).grid(row=0, column=1, padx=2, pady=2)
spin_header = Spinbox(root, from_=1, to=6,  width=5).grid(row=0, column=2, padx=2, pady=2)

b_ref = Button(root, text ="Referência").grid(row=1, column=1, padx=2, pady=2)
spin_href = Spinbox(root, from_=0, to=9999,  width=5).grid(row=1, column=2, padx=2, pady=2)

name_label = Label(root, text="Nome do Arquivo: ").grid(row=2, column=1, columnspan=2, padx=2, pady=2)
name_entry = Entry(root).grid(row=3, column=1, columnspan=2, padx=2, pady=2)
b_save = Button(root, text ="Finalizar e Salvar").grid(row=4, column=1, columnspan=2, padx=2, pady=2)

root.mainloop()

what I want to achieve is, when the b_header is pressed, html tags are added around whatever is selected in the text area (ie, add <h1> at the beggining of the selected text and </h1> at the end). if nothing is selected, just add the tags in the cursor position.
but currently I get this error when I press the button (the function addHeader is not working):

Error:
Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python3.7/tkinter/__init__.py", line 1705, in __call__
    return self.func(*args)
  File "teste.py", line 5, in addHeader
    a = main_text.get(Tk.SEL_FIRST, Tk.SEL_LAST)
AttributeError: 'NoneType' object has no attribute 'get'

any ideas on how to make it work?
thanks in advance