Access TextBox value in Gridview when OnTextChanged event triggered from another textbox

I am adding filters to a gridview.  One of the columns is a date field and I have 2 textboxes so the user can enter a range.  After the user enters the second date, the Event is triggered.   However, I can't access the data in the first text box.  I get an error that the object has not been set. This is the markup:   
I am adding filters to a gridview. One of the columns is a date field and I have 2 textboxes so the user can enter a range. After the user enters the second date, the Event is triggered. However, I can't access the data in the first text box. I get an error that the object has not been set. This is the markup: asp:TemplateField HeaderText="Open Date" SortExpression="RegistrationOpen"> <HeaderTemplate> <asp:LinkButton runat="server" ID="lbOpenDate" Text="Open Date& ...