I have a tkinter app. On load the focus is set to the first entry widget (self.entry_a). When I tab out of the entry (self.entry_a) for the first time it triggers the validation for the second entry (self.entry_b). This means that if I return False from the function validate_b, the application hangs

How can I fix this?

import tkinter as tk
from tkinter import ttk


class MainFrame():
    def __init__(self):
        self.root = tk.Tk()

        self.entry_value_a = tk.StringVar(value='0')
        self.entry_value_b = tk.StringVar(value='0')

        self.root.geometry('400x300')

        main_frame = self._main_frame()
        main_frame.grid(row=0, column=0, sticky=tk.EW)

        self.root.mainloop()

    def _main_frame(self) -> tk.Frame:
        frame = ttk.Frame(self.root)
        frame.columnconfigure(1, weight=1)

        valid_command_a = (self.root.register(self.validate_a), '%P')
        invalid_command_a = (self.root.register(self.on_invalid_a),)

        self.entry_a = ttk.Entry(frame, textvariable=self.entry_value_a)
        self.entry_a.config(
            validate='focusout',
            validatecommand=valid_command_a,
            invalidcommand=invalid_command_a
            )
        self.entry_a.grid(row=0, column=0)

        self.entry_a.focus_set()
        self.entry_a.select_range(0, 999)

        valid_command_b = (self.root.register(self.validate_b), '%P')
        invalid_command_b = (self.root.register(self.on_invalid_b),)

        self.entry_b = ttk.Entry(frame, textvariable=self.entry_value_b)
        self.entry_b.config(
            validate='focusout',
            validatecommand=valid_command_b,
            invalidcommand=invalid_command_b
            )
        self.entry_b.grid(row=1, column=0)

        self.label_error = ttk.Label(frame, foreground='red')
        self.label_error.grid(row=2, column=0, sticky=tk.W, padx=5)
        return frame

    def validate_a(self, event: object = '') -> bool:
        print('a triggered')
        self.label_error['text'] = ''
        if self.entry_value_a.get() == '0':
            return False
        return True

    def on_invalid_a(self):
        self.label_error['text'] = 'Invalid entry a'
        self.entry_a.focus_set()
        self.entry_a.select_range(0, 999)

    def validate_b(self, event: object = '') -> bool:
        print('b triggered')
        self.label_error['text'] = ''
        if self.entry_value_b.get() == '0':
            return  # False
        return True

    def on_invalid_b(self):
        self.label_error['text'] = 'Invalid entry b'
        self.entry_b.focus_set()
        self.entry_b.select_range(0, 999)


if __name__ == '__main__':
    MainFrame()