I don't see how cmbX is Nothing at this line,
Code:
cmbX.DataSource = desc
because before you reach that line you have this reference to cmbX,
Code:
cmbX.Items.Clear()
If cmbX is Nothing wouldn't this throw an error.

I'm not sure why your even passing cmbTax, if it's added to the form in the designer then it's accessible to all underlying code in the form.