@classmethod
def close_all(cls, strict=True):
    """
        Closes all Windows.

        Raises RuntimeError if `strict` is true and there are no open Windows.
        """
    if not cls._windows:
        if strict:
            raise RuntimeError('No windows.')
        return
    root = cls._windows[0]._tk_window
    root.destroy()
    for window in cls._windows:
        window._tk_window = None
        window.canvas = None
    cls._windows.clear()
