This is coming from MW 1.27, I also tried from MW 1.31.

Just an update for you all, using 1.35.5 (not 1.35.3 which still has this issue) and the latest PageForms 5.3.4, everything is working as expected.

@Aklapper thank you. We are doing an upgrade now. If this fixes the errors with PageForms, I will update this page.

I've updated to the latest 5.3.3 on MW 1.35.1 and disabled the extension GoogleMapsEditor (display a Google Maps for geography). The error persists, there is no callback to the Form page from the upload window dialogue; no file gets put into the form field when closing the blank white window.

You are correct, this is a different error but the end result is the same, no filename in written field:

after the upload dialogue, you come back to the form, where NO filename is written in the field
when you save the page, no edit has been done, thus no reference to the file is stored on the page

I am using MW 1.35.1, PF 4.9.4 since 5.3.3 does not have PFGoogleMapsInput::getName() which I use for my forms to display a GoogleMap to grab a WKT Geography. I even grabbed REL1_35 (4.9.4) from git but that has the $this-> error (replacing with wfMessage in those 2 functions (getInitialPageText and getExistsWarning) resolves that issue.
I guess maybe something else is broken since Edit with Form doesn't show up on a form created. https://imgur.com/a/zCBA4jP

Also on a MW 1.35 instance with REL_135 checked out.
Error with $this-> in getInitialPageText which should be using wfMessage