.net - Null exception was unhandled? -

i working on assignment friend. asked on how loop , gave me code part. copied , pasted vb. works him everytime try debug keep getting sign "null exception unhandled". not 1 one line. fist starts on lstinvoice.items.clear() if delete that, goes through lines. going on? used lstinvoice.items.clear() before on other assignments , never had problem before. here code:

private sub btnstraight_click(byval sender system.object, byval e system.eventargs) handles btnstraight.click         dim cost double          cost = txtcost.text         dim salvage_value double         salvage_value = 0         dim life double         life = txtlife.text         dim depreciation double         depreciation = (cost / life)         dim c integer, integer, x integer, y integer, z integer         c = cint(cdbl(txtyear.text))         = cint(txtlife.text)         x = cint(txtcost.text)         y = cint(cdbl(x) / i)         z = x - y         lstinvoice.items.clear()         lstinvoice.items.add("description: " & "" & txtdescription.text)         lstinvoice.items.add("year of purchase: " & txtyear.text)         lstinvoice.items.add("cost: " & formatcurrency(txtcost.text))         lstinvoice.items.add("estimated life:" & txtlife.text)         lstinvoice.items.add("method of depresciation: straight-line method")         lstinvoice.items.add("")         lstinvoice.items.add("value beginning of " & c & ": " & formatcurrency(cint(txtcost.text)))         lstinvoice.items.add("amount of depreciation accruing: " & c & ": " & formatcurrency(y))         lstinvoice.items.add("total depreaciation @ end of " & c & ": " & formatcurrency(z))         lstinvoice.items.add("")         c = c + 1         while (x > 0)             y = cint(cdbl(x) / i)             z = x - y             x = z             lstinvoice.items.add("value beginning of " & c & ": " & formatcurrency(x))             lstinvoice.items.add("amount of depreciation accruing: " & c & ": " & formatcurrency(y))             lstinvoice.items.add("total depreaciation @ end of " & c & ": " & formatcurrency(z))             lstinvoice.items.add("")             = - 1             c = c + 1         loop 

you haven't capitalized properly, should this:

lstinvoice.items.clear() 

notice capital , c.