#fixed code
                                public void flatFields() {
        if (document.isAppendMode()) {
            throw new PdfException(PdfException.FieldFlatteningIsNotSupportedInAppendMode);
        }
        Map<String, PdfFormField> fields = getFormFields();
        PdfPage page = null;
        for (Map.Entry<String, PdfFormField> entry : fields.entrySet()) {
            PdfFormField field = entry.getValue();

            PdfDictionary pageDic = field.getPdfObject().getAsDictionary(PdfName.P);
            if (pageDic == null) {
                continue;
            }

            page = getPage(pageDic);

            PdfDictionary appDic = field.getPdfObject().getAsDictionary(PdfName.AP);
            PdfObject asNormal = null;
            if (appDic != null) {
                asNormal = appDic.getAsStream(PdfName.N);
                if (asNormal == null) {
                    asNormal = appDic.getAsDictionary(PdfName.N);
                }
            }
            if (generateAppearance) {
                if (appDic == null || asNormal == null) {
                    field.regenerateField();
                    appDic = field.getPdfObject().getAsDictionary(PdfName.AP);
                }
            }
            if (appDic != null) {
                PdfObject normal = appDic.get(PdfName.N);
                PdfFormXObject xObject = null;
                if (normal.isStream()) {
                    xObject = new PdfFormXObject((PdfStream) normal);
                } else if (normal.isDictionary()) {
                    PdfName as = field.getPdfObject().getAsName(PdfName.AS);
                    xObject = new PdfFormXObject(((PdfDictionary)normal).getAsStream(as)).makeIndirect(document);
                }

                if (xObject != null) {
                    Rectangle box = field.getPdfObject().getAsRectangle(PdfName.Rect);
                    PdfCanvas canvas = new PdfCanvas(page);
                    canvas.addXObject(xObject, box.getX(), box.getY());
                    PdfArray fFields = getFields();
                    fFields.remove(field.getPdfObject().getIndirectReference());
                    PdfArray annots = page.getPdfObject().getAsArray(PdfName.Annots);
                    annots.remove(field.getPdfObject().getIndirectReference());
                    PdfDictionary parent = field.getPdfObject().getAsDictionary(PdfName.Parent);
                    if (parent != null) {
                        PdfArray kids = parent.getAsArray(PdfName.Kids);
                        kids.remove(field.getPdfObject().getIndirectReference());
                        if (kids == null || kids.isEmpty()) {
                            fFields.remove(parent.getIndirectReference());
                        }
                    }
                }
            }
        }

        if (page != null && page.getPdfObject().getAsArray(PdfName.Annots).isEmpty()) {
            page.getPdfObject().remove(PdfName.Annots);
        }

        getPdfObject().remove(PdfName.NeedAppearances);
        if (getFields().isEmpty()) {
            document.getCatalog().getPdfObject().remove(PdfName.AcroForm);
        }
    }
#vulnerable code 
                                public void flatFields() {
        if (document.isAppendMode()) {
            throw new PdfException(PdfException.FieldFlatteningIsNotSupportedInAppendMode);
        }
        List<PdfFormField> fields = getFormFields();
        PdfPage page = null;
        for (PdfFormField field : fields) {

            PdfDictionary pageDic = field.getPdfObject().getAsDictionary(PdfName.P);
            if (pageDic == null) {
                continue;
            }
            for (int i = 1; i <= document.getNumOfPages(); i++) {
                page = document.getPage(i);
                if (page.getPdfObject() == pageDic) {
                    break;
                }
            }

            PdfDictionary appDic = field.getPdfObject().getAsDictionary(PdfName.AP);
            PdfObject asNormal = null;
            if (appDic != null) {
                asNormal = appDic.getAsStream(PdfName.N);
                if (asNormal == null) {
                    asNormal = appDic.getAsDictionary(PdfName.N);
                }
            }
            if (generateAppearance) {
                if (appDic == null || asNormal == null) {
                    field.regenerateField();
                    appDic = field.getPdfObject().getAsDictionary(PdfName.AP);
                }
            }
            if (appDic != null) {
                PdfObject normal = appDic.get(PdfName.N);
                PdfFormXObject xObject = null;
                if (normal.isStream()) {
                    xObject = new PdfFormXObject((PdfStream) normal);
                } else if (normal.isDictionary()) {
                    PdfName as = field.getPdfObject().getAsName(PdfName.AS);
                    xObject = new PdfFormXObject(((PdfDictionary)normal).getAsStream(as)).makeIndirect(document);
                }

                if (xObject != null) {
                    Rectangle box = field.getPdfObject().getAsRectangle(PdfName.Rect);
                    PdfCanvas canvas = new PdfCanvas(page);
                    canvas.addXObject(xObject, box.getX(), box.getY());
                    PdfArray fFields = getFields();
                    fFields.remove(field.getPdfObject().getIndirectReference());
                    PdfArray annots = page.getPdfObject().getAsArray(PdfName.Annots);
                    annots.remove(field.getPdfObject().getIndirectReference());
                    PdfDictionary parent = field.getPdfObject().getAsDictionary(PdfName.Parent);
                    if (parent != null) {
                        PdfArray kids = parent.getAsArray(PdfName.Kids);
                        kids.remove(field.getPdfObject().getIndirectReference());
                        if (kids == null || kids.isEmpty()) {
                            fFields.remove(parent.getIndirectReference());
                        }
                    }
                }
            }
        }

        if (page.getPdfObject().getAsArray(PdfName.Annots).isEmpty()) {
            page.getPdfObject().remove(PdfName.Annots);
        }

        getPdfObject().remove(PdfName.NeedAppearances);
        if (getFields().isEmpty()) {
            document.getCatalog().getPdfObject().remove(PdfName.AcroForm);
        }
    } 
                            #location 64 
                            #vulnerability type NULL_DEREFERENCE