'/Fields' is a PDFRef so it cannot be pushed to

[bistrachilikova]

Describe the bug and the expected behaviour
We are attempting to sign a PDF file and we are adding a placeholder with placeholder-pdf-lib. However, we are getting the 'fields.push is not a function' error from pdfLibAddPlaceholder.js:124
fields.push(widgetDictRef);
I noticed that when reading the PDF, the '/Fields' object is a PDFRef, not a PDFArray - not sure if this is expected.

Is it a bug in signing or in the helpers?
Not sure

To Reproduce
I'm unfortunately unable to give you the PDF that's giving us issues. Here is what the /Fields object looks like:
PDFName { encodedName: '/Fields' } => PDFRef { objectNumber: 32, generationNumber: 0, tag: '32 0 R' },

[vbuch]

Just in theory (without looking at code), the PDFRef should be pointing towards an array object where it should be possible to push.