'Page' object has no attribute 'formats' when using mkdocs-print-site-plugin

[nbanyan]

I've also encountered this error with a very large MkDocs project I manage, but I haven't been able to find a minimal way to reproduce this error from that, although I suspect it may be a mismatch between MkDocs' Page collection and the nav list after some large scale of both.
However, enabling the mkdocs-print-site-plugin reliably creates this error.

I found that setting an initializer when formats.pdf.Plugin.on_nav() enumerates the flattened nav resolves this issue. I don't know enough about the rest of the formats code to know if this might cause side effects, but I haven't noticed any yet.

self.pages = flatten(nav)

for index, page in enumerate(self.pages):
  page.index = index
  page.formats = {}