title: Defining and adding value to variable from session-variable
body: <p>I'm trying to make a script that checks, if a user has the right age before joining a team. If the user age doesn't match the team age, the script should stop at this page, and require the user to click the button "BackToLastPageBtn" go back to the previous page, which uses a variable called "BackToLastPage", which gets its value from 'Session["currentUrl"]', before it is reset at Page_load.
  The problem is, that it tells me the value is null, when clicking the button.
  I don't know why it is null, when i add the value to "BackToLastPage", BEFORE resetting Session["currentUrl"]. I hope someone can tell me, and guide me in the right direction.</p>
  
  <p><strong>The CodeBehind - script</strong></p>
  
  <pre><code>public partial class JoinTeam : System.Web.UI.Page
  {
      //Defining Go back variable
      private string BackToLastPage;
  
      protected void Page_Load(object sender, EventArgs e)
      {
          int BrugerId = Convert.ToInt32(Session["BrugerId"]);
          int TeamId = Convert.ToInt32(Request.QueryString["HoldId"]);
  
          //Adding value to go back variable from sessionurl
          BackToLastPage = (string)Session["CurrentUrl"];
  
          //Resets sessionurl.
          Session["CurrentUrl"] = null;
  
          if (Session["brugerId"] != null)
          {
  
              if (ClassSheet.CheckIfUserAgeMatchTeamAge(BrugerId, TeamId))
              {
                  ClassSheet.JoinATeam(BrugerId, TeamId);
  
                  if (BackToLastPage != null)
                  {
                      //Uses the new savedUrl variable to go back to last page.
                      Response.Redirect(BackToLastPage);
                  }
                  else
                  {
                      Response.Redirect("Default.aspx");
                  }
              }
  
              else
              {
                  AgeNotOk.Text = "Du har ikke den rigtige alder til dette hold";
              }
          }
          else
          {
              //Not saving last page. Need to find solution.
              Response.Redirect("Login.aspx");
          }
      }
  
      //NOT WORKING...
      protected void BackToLastPageBtn_Click(object sender, EventArgs e)
      {
          //Go back button
  
          //Response.Write(BackToLastPage);
          Response.Redirect(BackToLastPage);
      }
  }
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/5an7F.gif" alt="Error Message"></p>