How to get the parent grid row number in nested gridview

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C#

I am working on a project in which i need to get parent gridview row on rowdatabound of child grid,i got the row number of parent from first child using code

string _ID = ((HiddenField)e.Row.Parent.Parent.Parent.FindControl("HdnID")).Value;

but this code is not working with the second child grid(I have three grids) it shows object reference error