pdf Java iTextPDF 7 NullPointerException位于PdfDocument。copyPagesTo

Question

当我想将页面从包含表的文档复制到另一个也包含表的文档时，会发生此错误。以下是代码示例：

ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
PdfWriter tempWriter = new PdfWriter(outputStream);
PdfDocument docFrom = new PdfDocument(tempWriter );
Document temp = new Document(docFrom);
Table table = new Table(1);

for (News singleNews : news) {
  table.addCell(cellFrom(singleNews));
}

temp.add(table);
temp.flush();
doc.copyPagesTo(1,doc.getNumberOfPages(),document);

抛出错误堆栈跟踪：

java.lang.NullPointerException
    at com.itextpdf.kernel.pdf.PdfDictionary.copyTo(PdfDictionary.java:535)
    at com.itextpdf.kernel.pdf.PdfPage.copyTo(PdfPage.java:373)
    at com.itextpdf.kernel.pdf.PdfDocument.copyPagesTo(PdfDocument.java:1111)
    at com.itextpdf.kernel.pdf.PdfDocument.copyPagesTo(PdfDocument.java:1033)
    at com.itextpdf.kernel.pdf.PdfDocument.copyPagesTo(PdfDocument.java:1064)
    at com.itextpdf.kernel.pdf.PdfDocument.copyPagesTo(PdfDocument.java:1047)