Bug report

Bug summary

A recent bugfix (#17102) in the Tk backend created a new IntVar that does not make reference to a parent widget.

Code for reproduction

import tkinter
from matplotlib.figure import Figure
from matplotlib.backends.backend_tkagg import NavigationToolbar2Tk, FigureCanvasTkAgg

tkinter.NoDefaultRoot()
window = tkinter.Tk()
fig = Figure()
canvas = FigureCanvasTkAgg(fig, window)
navbar = NavigationToolbar2Tk(canvas, window)

Actual outcome

NameError: name '_default_root' is not defined

Expected outcome

no crashes

Matplotlib version

• Operating system: Win10
• Matplotlib version: 3.3.1
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 3.8.5
• Jupyter version (if applicable):
• Other libraries: