setting textbox to property throw null ref exception

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C#

ASP.NET

I have a form in which there is a textbox

<asp:textbox id="txtbox1" runat="server">

in the code behind i set of the asp.net page

class1.class2.class2txt1= txtbox1.text.trim();


public class1
{
private int _val;
private class2 _class2val;

public int VAl
{
get {return this._val;}
set {this._val = value;}

}
public class2 Class2Val
{
get {return this._class2val;}
set {this._class2val= value;}
}

public class2
{
private string _txtvalue=string.empty;

public string class2txt1
{
get {return this._txtvalue;}
set {this._txtvalue= value;}
}
}

Im relatively new to .net ...please help as to why am i getting a nullreference exception when i postback .