NullReferenceException while Im trying to pass parameters from one page to another

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.NET3.5

C#

ASP.NET

.NET

exceptions

I would be grateful if you could help me to solve the problem. Hello I try to pass some value from one page to another but at the run time i get exception.

Here is my ASP code:

Enter a value to post: 
  <asp:textbox id="TextBox1" runat="Server"> 
  </asp:textbox>  
<br />
<br /> 
  <asp:button id="Button1" text="Post back to this page" runat="Server">      
  </asp:button>  
<br />
<br />  
  <asp:button id="Button2" text="Post value to another page" 
  postbackurl="PostBackUrlPage2cs.aspx" runat="Server"> 
  </asp:button>

Here's the code-behind on the destination page:

void Page_Load(object sender, System.EventArgs e)    
 {        
  string text;          
  // Get the value of TextBox1 from the page that          
  // posted to this page.         
  text = ((TextBox)PreviousPage.FindControl("TextBox1")).Text;          
  // Check for an empty string.         
 if (text != "")            
  PostedLabel.Text="The string posted from the previous page is "+text+".";       
 else             
  PostedLabel.Text = "An empty string was posted from the previous page.";     
 } 

I get this Exception:

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

I took the example from msdn:
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.button.postbackurl.aspx

Why do I get this exception?and how can I solve this problem?

Thank you in advance.