Unable to flatten PDF if widget doesn't have P reference

When running form.flatten() on some PDFs, like from this this example, I get this error:

 Error: Failed to find page undefined for element favorite.superhero
    at _loop_1 (/node_modules/pdf-lib/cjs/api/form/PDFForm.js:463:27)
    at PDFForm.flatten (/node_modules/pdf-lib/cjs/api/form/PDFForm.js:492:17)

Here is an example of PDF that will produce this error and where form isn’t on 1st page. (Originally posted here)

[btecu]

I’ve created a draft pull request with a possible solution. The approach was to find the widget reference and then find the page that has that widget reference. Ideally, I would have preferred to update P() to do all this work, so that we don’t have to call another method for fallback, however, the document is required to get the pages and I’m not aware if there’s a way to access the document without passing it in. Perhaps @Hopding has other ideas or opinions about the apis?

However, when testing the example document, I am getting a different error, possibly unrelated, something about AcroField. @Hopding / @chislin do you have some time to take a look?

[btecu]

@Hopding thanks for the detailed explanation. The pull request should be ready for review now.