Space in check box export value

[cbgithub2]

check box export value with space set_field threw error message.

Unhandled Java Exception in create_output():
java.lang.ClassCastException: pdftk.com.lowagie.text.pdf.PdfNull cannot be cast to pdftk.com.lowagie.text.pdf.PdfDictionary
at pdftk.com.lowagie.text.pdf.FdfReader.readFields(FdfReader.java:146)
at pdftk.com.lowagie.text.pdf.FdfReader.readPdf(FdfReader.java:117)
at pdftk.com.lowagie.text.pdf.PdfReader.(PdfReader.java:172)
at pdftk.com.lowagie.text.pdf.PdfReader.(PdfReader.java:161)
at pdftk.com.lowagie.text.pdf.FdfReader.(FdfReader.java:74)
at com.gitlab.pdftk_java.TK_Session.create_output(TK_Session.java:2717)
at com.gitlab.pdftk_java.pdftk.main_noexit(pdftk.java:156)
at com.gitlab.pdftk_java.pdftk.main(pdftk.java:128)
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 (3.0.6), and if possible steps to reproduce the error.