I created a button, to dynamically create a validation on cell

the following is the code i wrote:

......

Excel.Range thisRange =
((Excel.Worksheet)(this.thisWorkbook.Worksheets.ge t_Item("XXXXX"))).get_Range("A1", Type.Missing);

thisRange.Validation.Add(Excel.XlDVType.xlValidate List,
Excel.XlDVAlertStyle.xlValidAlertStop,
Excel.XlFormatConditionOperator.xlBetween,"Yes, No", Type.Missing);

......

I tested, and found that thisRange is not null and thisRange.Validation is
not null either. However, when i wrong the code, and clicked the button, it
tells me "Object Reference is Not set to an instance of an object".

But the range object is not, while validation is not null either. Can anyone
help me solve this problem?

Many thanks