insertText/HTML - object is null for ASP.NET/AJAX

I am a new user of the ckeditor and I am running into an issue with using the insertText and insertHTML menthods. I have an ASP.NET 4.0 webpage using an UpdatePanel. The CKEditor within the updatepanel/ContentTemplate tags. I have an button control with an OnClientClick event to a javascript function:

    function InsertTest() {
        var oEditor = CKEDITOR.instances.MainContent_CKEditor1;
        var value = 'This is plain text';
        // Check the active editing mode.
        if (oEditor.mode == 'wysiwyg') {
            oEditor.insertText(value);
        }
        else
            alert('You must be in WYSIWYG mode!');
    }

During the postback, I get:

Microsoft JScript runtime error: Unable to get the value of the property 'getFrameDocument': object is null or undefined

The editor has this line highlighted:

getSnapshotData:function(){return M.getFrameDocument().getBody().getHtml();

When I hover of the "M.", its null.

The editor works when used without the updatepanel.

Any help would be appreciated.