Problem With ajaxtoolkit:combobox c#

hello guys :

I have combobox (typeco) witch it's bounded with table (type) .. and another combobox (modcom) bounded with table (model) ... when I choose the type , all models are changing the value of (modcom) by (typeco) ... this is not my problem .... my problem is : when I try to write not exist item it give me error directly ... i need to add items through a button but it shows this error ... i've try try catch and didn't work .... try add error in global.asax and it redirect me into other page ... but I want to add this new item via button at the same page !! ... I can't disable autopostback ....
note : typeco and modcom  bounded to sqlconnection by GUI configuration
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