How to recover from the error?

Question

I am retrieving data from database and bind to a gridview & listview controls and here i need to display each "tags"(i.e, comma separated values) as hyperlinks.

string tags = string.Empty;
using (DataClass1DataContext DcContext = new DataClass1DataContext())
{
GridView1(or)ListView1.DataSource = DcContext.tblArticles
.Join(DcContext.tblCategories,
x => x.category,
y => y.categoryId,
(x, y) => new { x, y })
.Join(DcContext.tblSubCategories,
p => p.x.subcategory,
q => q.subcategoryId,
(p, q) => new
{
tags = p.x.keywords,
Article = p.x.articleId,
Published = p.x.articleDate,
Title = p.x.title,
Summary = p.x.description,
Labels = p.x.keywords,
Poster = p.x.image,
Category = p.y.Category,
Subcategory = q.subcategory,
});

What I need is?
The tags which i retrieved will need to display as related keywords in the form of hyperlinks and bind to the gridview/Listview.

 

<asp:GridView ID="GridView1" runat="server" Width="100%" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:Image ID="PosterImg" runat="server" ImageUrl='<%#Eval("Poster") %>' Width="50" Height="50" />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="Published" HeaderText="Published on" />
<asp:BoundField DataField="Title" HeaderText="Article" />
<asp:BoundField DataField="Summary" HeaderText="Summary about your articles" />
<asp:TemplateField>
<ItemTemplate>
<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="Category" HeaderText="Category" />
<asp:BoundField DataField="Subcategory" HeaderText="Subcategory" />
</Columns>
</asp:GridView>
<asp:ListView ID="ListView1" runat="server">
<ItemTemplate>
<asp:Image ID="PosterImg" CssClass="thumb" runat="server" ImageUrl='<%#Eval("Poster") %>' Width="120" Height="150" />
<h4>
<asp:HyperLink ID="HyperLink2" NavigateUrl='<%#"?p="+Eval("Article") %>' runat="server"><%#Eval("Title") %></asp:HyperLink>
</h4>
<cite>
Posted by
<asp:HyperLink ID="HyperLink3" NavigateUrl='<%#"?user="+Eval("Article") %>' runat="server">Username</asp:HyperLink>
under
<span>
<asp:HyperLink ID="HyperLink4" NavigateUrl='<%#"?subcategory="+Eval("Subcategory") %>' runat="server"><%#Eval("Subcategory") %></asp:HyperLink>
·
<asp:HyperLink ID="HyperLink5" NavigateUrl='<%#"?category="+Eval("Category") %>' runat="server"><%#Eval("Category") %></asp:HyperLink>
</span>
on
<time><%#Eval("Published") %></time>
</cite><br />
<p>
<%#Eval("Summary") %>
</p>
<span>
<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
</span>
</ItemTemplate>
</asp:ListView>

 

What I have tried:

Added a placeholder inside the item template of each control(i.e, GRIDVIEW and LISTVIEW)
and by using LINQ I retrieved it from database and assign it to local string variable.

 

string[] strArray = tags.Split(',');
foreach (var obj in strArray)
{
HyperLink TagLinks = new HyperLink();
TagLinks.NavigateUrl = "?tags=" + obj;
TagLinks.Text = obj;
PlaceHolder PlaceHolder1 = (PlaceHolder)GridView1.FindControl("PlaceHolder1");
PlaceHolder1.Controls.Add(TagLinks);
}

I received an error:

Object reference not set to an instance of an object.

How to resolve this?