sap b1 problem

Hi,

We are trying to customizing the Sapb12005b

i want to take the input value given by the user from my Addon.

i put one Combobox on my addon, i want to bring data from database by using that combobox selected value.

i am using vb.net, in that i am getting the selected value like

"combo.selected.value"

but i am getting error like Null reffrence exception. plz help me. asap

Regards,

nagababu