The purpose of focus_get() is to return the widget that currently has the focus. It tries to convert its result to a tkinter widget, which can fail, because not all Tk widgets are known to tkinter. Consider this, for example:

    import tkinter

    def print_focused_widget():
        print(repr(root.focus_get()))
        root.after(1000, print_focused_widget)

    root = tkinter.Tk()
    menu = root["menu"] = tkinter.Menu()
    menu.add_cascade(label="Click here", menu=tkinter.Menu())
    print_focused_widget()
    tkinter.mainloop()

Output, with menu clicked after a couple seconds (on Linux):

    None
    <tkinter.Tk object .>
    <tkinter.Tk object .>
    Exception in Tkinter callback
    Traceback (most recent call last):
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 1916, in __call__
        return self.func(*args)
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 838, in callit
        func(*args)
      File "/home/akuli/porcu/foo.py", line 4, in print_focused_widget
        print(repr(root.focus_get()))
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 782, in focus_get
        return self._nametowidget(name)
      File "/home/akuli/.local/lib/python3.10/tkinter/__init__.py", line 1531, in nametowidget
        w = w.children[n]
    KeyError: '#!menu'

Some nametowidget() calls in tkinter/__init__.py already handle this correctly. Consider winfo_children(), for example:

            try:
                # Tcl sometimes returns extra windows, e.g. for
                # menus; those need to be skipped
                result.append(self._nametowidget(child))
            except KeyError:
                pass