What's happening here is that you have a menu item with a default image for its action, and you're assigning nil to the image property of the item before the menu is ever displayed.
Normally, when an app assigns a value to a menu item's image property, AppKit will remove the "has a default image" state from the menu item. However, AppKit has a special case for this particular path, where nil is assigned prior to the menu being displayed. In this case, the "has a default image" state is not removed, and the menu item retains its default image. This is meant to improve compatibility with some apps that create a new item and immediately set the image to nil unconditionally. Without this special-case behavior, these apps would not get the default image at all. The special-case behavior allows these apps to still get the default image for their item actions.
In your case, the recommended solution would be to set the menu item's image to a non-nil image object first, and then immediately set to nil. That will remove the default menu item image.
