Created attachment 31338 [details]
Template file to reproduce the issue

I'm trying to add rows to an Excel 2010 table that has no visible header rows. However, POI-3.10 throws an exception on safe (see below).

Given:
======
an Excel 2010 file that contains a table with no visible header rows

Description:
============
New rows are added to the table and the table is expanded before safe. POI tries to update the table headers in org.apache.poi.xssf.usermodel.XSSFTable#updateHeaders but fails to take into account that there are no visible table headers. 

If I use the debugger to set the value of row in XSSFTable line 295 to null, the exception can be avoided and POI produces a file that opens in Excel without errors. I guess adding a condition that checks for number of visible header rows should help. 

Please note:
This exception does not occur if the table has headers. It also does not occur if there is no other content in the file (thus the x in column C).

Demo program to reproduce (please adjust paths before running):
===============================================================
public class Demo
{
	public static void main(String[] args) throws IOException, InvalidFormatException
	{
		final Workbook workbook = WorkbookFactory.create(new File("Tabelle.xlsx"));
		final XSSFSheet sheet = (XSSFSheet) workbook.getSheetAt(0);

		// add some contents to table so that the table will need expansion
		Row row = sheet.getRow(0);
		Cell cell = row.createCell(0);
		cell.setCellValue("demo1");
		cell = row.createCell(1);
		cell.setCellValue("demo2");
		cell = row.createCell(2);
		cell.setCellValue("demo3");

		row = sheet.getRow(1);
		cell = row.createCell(0);
		cell.setCellValue("demo1");
		cell = row.createCell(1);
		cell.setCellValue("demo2");
		cell = row.createCell(2);
		cell.setCellValue("demo3");

		// expand table
		XSSFTable table = sheet.getTables().get(0);
		final CellReference startRef = table.getStartCellReference();
		final CellReference endRef = table.getEndCellReference();
		table.getCTTable().setRef(new CellRangeAddress(startRef.getRow(), 1, startRef.getCol(), endRef.getCol()).formatAsString());


		FileOutputStream stream = new FileOutputStream("e:\\output.xlsx");
		workbook.write(stream);
		stream.close();
	}

}

Excpetion:
==========
Exception in thread "main" org.apache.xmlbeans.impl.values.XmlValueDisconnectedException
	at org.apache.xmlbeans.impl.values.XmlObjectBase.check_orphaned(XmlObjectBase.java:1213)
	at org.openxmlformats.schemas.spreadsheetml.x2006.main.impl.CTCellImpl.getF(Unknown Source)
	at org.apache.poi.xssf.usermodel.XSSFCell.getCellType(XSSFCell.java:529)
	at org.apache.poi.xssf.usermodel.XSSFCell.getRichStringCellValue(XSSFCell.java:264)
	at org.apache.poi.xssf.usermodel.XSSFCell.getStringCellValue(XSSFCell.java:251)
	at org.apache.poi.xssf.usermodel.XSSFTable.updateHeaders(XSSFTable.java:301)
	at org.apache.poi.xssf.usermodel.XSSFTable.writeTo(XSSFTable.java:84)
	at org.apache.poi.xssf.usermodel.XSSFTable.commit(XSSFTable.java:95)
	at org.apache.poi.POIXMLDocumentPart.onSave(POIXMLDocumentPart.java:322)
	at org.apache.poi.POIXMLDocumentPart.onSave(POIXMLDocumentPart.java:326)
	at org.apache.poi.POIXMLDocumentPart.onSave(POIXMLDocumentPart.java:326)
	at org.apache.poi.POIXMLDocument.write(POIXMLDocument.java:173)
	at Demo.main(PoiBug.java:49)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)