Disable bottom_toolbar if it returns False

I want to add shortcut to disable bottom toolbar. Is it possible to disable it when bottom_toolbar returns False?

#!/usr/bin/env python3

import os
from prompt_toolkit import PromptSession
from prompt_toolkit.key_binding import KeyBindings

global_kb = KeyBindings()
global_toolbar = True

def bottom_toolbar():
    global global_toolbar
    if global_toolbar:
        return "F1 Help"
    else:
        return False

@global_kb.add('c-b')  # Ctrl+B
def switch_toolbar(event):
    global global_toolbar
    global_toolbar = not global_toolbar

session = PromptSession(os.getcwd() + ">", key_bindings=global_kb, bottom_toolbar=bottom_toolbar)
text = session.prompt()

print("[debug] " + text)

Right now this piece of code enters endless error loop on Ctrl+B:

Exception No formatted text. Expecting a unicode object, HTML, ANSI or a FormattedText instance. Got False

Full traceback: https://pastebin.mozilla.org/9088696

Start with the handling of the bottom_toolbar callback and reproduce the supplied PromptSession example. Trigger Ctrl+B while the callback returns False, then verify that the toolbar is hidden without raising the formatted-text error or entering an error loop.