NullPointerException in SimpleBookmark.getBookmark()

[andreasrosdal]

Describe the bug
We get an NullPointerException in SimpleBookmark.java SimpleBookmark.getBookmark().
OpenPDF version 1.0.5. Running on Linux with Java 11.

This is the Exception stacktrace:
Caused by: java.lang.NullPointerException
at com.lowagie.text.pdf.SimpleBookmark.bookmarkDepth(SimpleBookmark.java:123)
at com.lowagie.text.pdf.SimpleBookmark.getBookmark(SimpleBookmark.java:289)
at no.foobar.pdf.PdfDocument.write(PdfDocument.java:605)

This is the relevant code line where the NullPointerException occurs:
https://github.com/LibrePDF/OpenPDF/blob/1.0.5/openpdf/src/main/java/com/lowagie/text/pdf/SimpleBookmark.java#L123

To Reproduce
Code to reproduce the issue
List bookmarks = SimpleBookmark.getBookmark(reader);