Handle Null condition

Hi Experts,

I want to handle null condition in below code.

lstTest.Discount = If((Not dataSet.Tables("History") Is Nothing), If(IsDBNull(dataSet.Tables("History").Rows(0)("DiscountsAdjustmentsAmount")), "$0.00", StringToCurrency(GetContractualDiscount(dataSet.Tables("History").Rows(0)("DiscountsAdjustmentsAmount"), dataSet.Tables("History").Rows(0)("DiscountsAdjustments"), dataSet.Tables("History").Rows(0)("EstimatedCharges")))), "$0.00")

My code is getting break at 

dataSet.Tables("History").Rows(0)("DiscountsAdjustments")

since its value is null. 

Please help how can I handle.

Thanks,
Rahul