Julien Nguyen

### Description A nullpointerexception is thrown when templating. ``` java.lang.NullPointerException at org.apache.poi.hssf.record.CFHeaderBase.serialize(CFHeaderBase.java:144) at org.apache.poi.hssf.record.StandardRecord.serialize(StandardRecord.java:56) at org.apache.poi.hssf.usermodel.HSSFWorkbook$SheetRecordCollector.serialize(HSSFWorkbook.java:1476) at org.apache.poi.hssf.usermodel.HSSFWorkbook.getBytes(HSSFWorkbook.java:1523) at org.apache.poi.hssf.usermodel.HSSFWorkbook.write(HSSFWorkbook.java:1418) at org.apache.poi.hssf.usermodel.HSSFWorkbook.write(HSSFWorkbook.java:1404) at org.jxls.transform.poi.PoiTransformer.writeButNotCloseStream(PoiTransformer.java:501) at org.jxls.transform.poi.PoiTransformer.write(PoiTransformer.java:482) at org.jxls.util.JxlsHelper.processTemplate(JxlsHelper.java:288) at org.jxls.util.JxlsHelper.processTemplate(JxlsHelper.java:253)...