Cannot add Rubberduck menu item to custom bar

[WoHuy]

Rubberduck version information
Version 2.5.9.6291
OS: Microsoft Windows NT 10.0.19045.0, x64
Host Product: Microsoft Office 2016 x86
Host Version: 16.0.5404.1000
Host Executable: WINWORD.EXE

Description
After moving a Rubberduck menu item to a custom toolbar, I get the following error message (repeatedly)

Upon exiting the application, this message is thrown:

To Reproduce
Steps to reproduce the behavior:

1. Right-clic on the toolbar area + Customize
2. Go to Toolbars tab + New to add a custom toolbar
3. With the Customize dialog still open, open the Rubberduck menu and a slide a menu option to the newly created toolbar (I moved Code Explorer from the Windows submenu)
4. Close the Customize dialog
5. Any new action (e.g. Refreshing the parser state) triggers the above errors

The only way out of this, is closing the entire application and ending the WINWORD.EXE process that lingers after that.

Expected behavior
The menu item is moved to the button bar.

Logfile
This logfile capture steps-to-reproduce 3 to 5:
RubberduckLog.txt

[MDoerner]

The exception on shutdown is the same as in #6150. I think that is a separate issue.