Error when apply the option flatten

Hello @All,

Thank you for developing such a great tool.

We use version 3.3.2 installed as a package on Debian and sometimes we have an error when applying the option flatten

Unhandled Java Exception in create_output():
java.lang.NullPointerException
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.PdfStamperImp.flatFields(PdfStamperImp.java:765)
	at com.gitlab.pdftk_java.com.lowagie.text.pdf.PdfStamperImp.close(PdfStamperImp.java:163)
	at com.gitlab.pdftk_java.filter.create_output_filter(filter.java:441)
	at com.gitlab.pdftk_java.TK_Session.create_output(TK_Session.java:1457)
	at com.gitlab.pdftk_java.pdftk.main_noexit(pdftk.java:190)
	at com.gitlab.pdftk_java.pdftk.main(pdftk.java:161)
There was a problem with pdftk-java. Please report it at
https://gitlab.com/pdftk-java/pdftk/issues
including the message above, the version of pdftk-java (3.3.2), and if possible steps to reproduce the error.

Thanks