Web Forms :: Unable To Access Hidden Textbox Value In Webpart On Aspx Page

I have a requirement where I have a webpart and two ImageButtons(imgGV, imgLV) and a hidden input textbox('ctl00$MainHeaderContent$txtType') in an aspx page. I need to change templates to this Webpart either to show results in "Grid View" or as "List View" on the click of this ImageButtons. I added code in the page_load of .aspx page as

imgLV.Attributes.Add("onclick", "document.getElementById('ctl00$MainHeaderContent$txtType').value='LV';return true;");
imgGV.Attributes.Add("onclick", "document.getElementById(('ctl00$MainHeaderContent$txtType')).value='GV';return true;");

When I am clicking the specific ImageButton, it has to write into hidden field either "GV" or "LV". In the webpart CreateChildControls I assign templates based on the text in Hidden field either "GV" (Grid Template) else "LV" (List Template). In the CreateChildControls of Webpart for the template change, I am getting the Hidden value as empty. I am using coding as

HtmlInputHidden t = new HtmlInputHidden();
t = (HtmlInputHidden)this.Page.FindControl("ctl00$MainHeaderContent$txtType");
if (t.Value=="GV") {
assign one template}
else { another template}.

I am getting t.Value="". Question is: Is it like if we add some text in a hidden textbox using the Javascript function in an aspx page, the same textbox value in the same aspx page but in the Webpart class in the CreateChildControls method using the code as this(webpart).page.FindControl("Textbox").text is showing empty. So the text added using Javascript into Textbox is it not available in the code behind code using "Text" property of the same Textbox?.