I found the issue when calling the Demo() method a second time. The first step is to create the workbook reference:

lnWb = this.CreateWorkbook("ExcelTest.xlsx")

This returns a 'handle' to the workbook object (1 based index) that is created. If it fails or if the same workbook already exists, then 0 is returned to denote a failure. So, executing Demo() a second time this method returns 0 (no workbook created). I did not include any checking for success in the demo. I have now added the following:

lnWb = this.CreateWorkbook("ExcelTest.xlsx")
IF lnWb = 0
	RETURN False
ENDIF