This question is asked every day, please do basic research such as using google before you ask a question.
gvWarden.HeaderRow.Style.Add("background-color", "#FFFFFF");
Either gWarden is null, HeaderRow is null, or Style is null. We can't run your code so can't tell you which, and can't tell you why. Use the debugger to find out what is null, then try and work out why it is null.