Microsoft JScript runtime error: 'null' is null or not an object

Question

I have application like where i can create dynamic tabs. and delete cross bar option on tabs. When I am trying to delete the tab I am getting error like

Microsoft JScript runtime error: 'null' is null or not an object and point to my Javascript code.

Here is my JS code.

  <script type="text/javascript">
        /* <![CDATA[ */
        function deleteTab(tabText)
        {
            var tabStrip = $find("<%= RadTabStrip1.ClientID %>");
            var multiPage = $find("<%= RadMultiPage1.ClientID %>");
            var tab = tabStrip.findTabByText(tabText);
            var pageView = tab.get_pageView();

            var tabToSelect = tab.get_nextTab();
            if (!tabToSelect)
                tabToSelect = tab.get_previousTab();

            tabStrip.get_tabs().remove(tab);
            multiPage.get_pageViews().remove(pageView);

            if (tabToSelect)
                tabToSelect.set_selected(true);
        }
        /* ]]> */
</script>

and in page lode

    if (!Page.IsPostBack)
    {
        RadTab tab = new RadTab();
        tab.Text = string.Format("New Page {0}", 1);
        RadTabStrip1.Tabs.Add(tab);

        RadPageView pageView = new RadPageView();
        pageView.Height = new Unit("50px");
        pageView.Width = new Unit("1300px");
        RadMultiPage1.PageViews.Add(pageView);

        BuildPageViewContents(pageView, RadTabStrip1.Tabs.Count);
        RadTabStrip1.SelectedIndex = 0;

    }

Answer 1

You're not checking any of those function calls to see if they're actually returning something. One of them is returning null, but your code does not notice that and tries to use the result in a subsequent statement.

Try this in Firefox with Firebug and you'll probably get better error messages.

Answer 2

This error can occur if you are trying to use an object which is null. In that code quite a lot of things can return null: $find, findTabByText, getPageView, get_nextTab, get_previousTab etc. I suggest you alert() everything before using it. That way you will find what is null.

Answer 3

$find can return null if you are trying to call it too early. Remember that ASP.NET AJAX controls are created during the Sys.Application.Init event. If you try to access them earlier (e.g. in the window.onload) the $find() will not work.