Unset keybinding toggle_scrollbar prevents right-click menu from opening

[JohelEGP]

Describe the bug
With the config file

[keybindings]
  toggle_scrollbar = None

performing a right-click on a terminal body doesn't open the menu and prints

Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/terminatorlib/terminal.py", line 1031, in on_buttonpress
    right_click[0](*right_click[1])
  File "/usr/lib/python3.10/site-packages/terminatorlib/terminal.py", line 1077, in popup_menu
    menu.show(widget, event)
  File "/usr/lib/python3.10/site-packages/terminatorlib/terminal_popup_menu.py", line 270, in show
    item = self.menu_item(Gtk.CheckMenuItem, 'toggle_scrollbar',
  File "/usr/lib/python3.10/site-packages/terminatorlib/terminal_popup_menu.py", line 55, in menu_item
    mask    = self.get_menu_item_mask(maskstr)
  File "/usr/lib/python3.10/site-packages/terminatorlib/terminal_popup_menu.py", line 33, in get_menu_item_mask
    maskstr = maskstr.lower()
AttributeError: 'NoneType' object has no attribute 'lower'

To Reproduce
Steps to reproduce the behavior:

1. Save the config file above.
2. Run terminator -g config-file
3. Right click the body of the terminal.

Expected behavior
The right-click menu opens.

Desktop (please complete the following information):

• OS or Linux Distribution: Arch Linux
• Version latest
• Display Technology: X11