Notice: Undefined offset: 1 in menu_editor_node_creation_form_alter()

The whitespace is a habit of mine. Always want to give the blank lines full indentation.
Is there a documented style rule that handles whitespace? Otherwise I would try to just keep the module consistent.

For the rest:
I didn't know that list($a,$b) is that strict. Error reporting level? Would it help to put a "@" ?
Your patch looks like a reasonable solution.

That said, I do have a local version somewhere which stores these values in the $form array instead of a static var. Seems like a better idea. I should commit this thing to -dev.

I committed a minimal fix to the 6.x-2.2 stable release.

The local version with hook_nodeapi and more changes will go into 6.x-2.3
This way, people can revert to 6.x-2.2, if 2.3 should cause any unforeseen issues.