Convert ODS to PDF (odftoolkit)

Saya ada masalah dalam menconvert ods file ke pdf file, akan tetapi ketika di running code-nya, saya mendapat balikan error : org.odftoolkit.odfdom.converter.core.ODFConverterException: java.lang.NullPointerException

kalau ada yang pernah mengalami dan tahu solusinya, monggo di share gan. Codenya saya coba lampirkan dibawah ini:

package odfdom;

import java.io.FileOutputStream;
import org.odftoolkit.odfdom.converter.pdf.PdfConverter;
import org.odftoolkit.odfdom.converter.pdf.PdfOptions;
import org.odftoolkit.odfdom.doc.OdfSpreadsheetDocument;

public class odfdomtest {
   private final static String SS_FINAL_FILE = "sheetFile.ods";
   private final static String SS_PDF_FILE = "pdfFile.pdf";
   private final static String AMOUNT_PATTERN = ":amount:";
   private float amount;

   public static final void main(String[] args) {
		new odfdomtest(25);
   }

   public odfdomtest(float amount) {
		processSpreadsheetFile();
   }

   public void processSpreadsheetFile() {
      OdfSpreadsheetDocument doc;
      try {
          doc = (OdfSpreadsheetDocument) OdfSpreadsheetDocument.loadDocument(SS_FINAL_FILE);
			PdfConverter.getInstance().convert(
					doc,
					new FileOutputStream(SS_PDF_FILE), PdfOptions.getDefault());
      } catch (Exception e) {
	e.printStackTrace();
      }
   }
}