How to display NULL values in grid view?

How to display NULL values in grid view? I am using Kendo grid view and it works great if I don't have NULL values that are joined to some foreign key (for example: _UnitKey). Thank You guys

Here is part of my odata file that is fetched from local server:

"_Key":11000002100000003,"SAPStorageLocation":"Kopenhagen","_OdsIntRefKey":null,"_UnitKey":81200000001,"_ProcessProductionRequestKey":null,"_MaterialKey":83000000004,"UnitOfMeasure":"hl","Status":0,"StartTime":"2015-02-03T11:56:05.39+01:00","Quantity":17.926000000,"ProductionResourceOrLine":"ProductionResourceOrLine","OrderType":"OrderType 20","MaterialLotID":"MaterialLotID 20","ID":"ID 20","CreationTime":"2015-02-03T11:56:05.39+01:00","Comment":"123456789012345678901234567890123456789

I would like to display if for example _UnitKey is not a number, if it is null..but it doesn`t display, I get a message:"Cannot read property '_UnitKey' of null"