Object reference not set to an instance of an object-exception - asp.net

Question

I have a panel with ID:Panel2 in my web page and I'm adding adynamic table into it.
I'm getting an "Object reference not set to an instance of an object" exception.
My code:
for (int i = 0; i <= val; i++)
{
Table Table4 = new Table();
TableHeaderRow thr = new TableHeaderRow();
TableHeaderCell thc2 = new TableHeaderCell();
thc2.BorderColor = Color.Black;
thc2.BorderWidth = 2;
TableHeaderCell thc3 = new TableHeaderCell();
thc3.BorderColor = Color.Black;
thc3.BorderWidth = 2;
TableHeaderCell thc4 = new TableHeaderCell();
thc4.BorderColor = Color.Black;
thc4.BorderWidth = 2;
TableHeaderCell thc5 = new TableHeaderCell();
thc5.BorderColor = Color.Black;
thc5.BorderWidth = 2;
Label l2 = new Label();
l2.Text = "Check Point";
l2.ForeColor = Color.Black;
Label l3 = new Label();
l3.Text = "Applicability";
l3.ForeColor = Color.Black;
Label l4 = new Label();
l4.Text = "Effectiveness";
l4.ForeColor = Color.Black;
Label l5 = new Label();
l5.Text = "Score";
l5.ForeColor = Color.Black;
thc2.Controls.Add(l2);
thc3.Controls.Add(l3);
thc4.Controls.Add(l4);
thc5.Controls.Add(l5);
thr.Cells.Add(thc2);
thr.Cells.Add(thc3);
thr.Cells.Add(thc4);
thr.Cells.Add(thc5);
Table4.Rows.Add(thr);
TableRow tr = new TableRow();
tr.BorderColor = Color.Black;
tr.BorderWidth = 2;
TableCell c2 = new TableCell();
c2.BorderColor = Color.Black;
c2.BorderWidth = 2;
c2.Text = Convert.ToString(s[i]);
TableCell c3 = new TableCell();
c3.BorderColor = Color.Black;
c3.BorderWidth = 2;
TableCell c4 = new TableCell();
c4.BorderColor = Color.Black;
c4.BorderWidth = 2;
TableCell c5 = new TableCell();
c5.BorderColor = Color.Black;
c5.BorderWidth = 2;
DropDownList ddl1 = new DropDownList();
ddl1.Items.AddRange(items2);
ddl1.ID = "ddl1" + i;
DropDownList ddl2 = new DropDownList();
ddl2.Items.AddRange(items1);
ddl2.ID = "ddl2" + i;
TextBox t4=new TextBox();
t4.ID = "textID4" + i;
t4.EnableViewState = true;
c2.ID = "newC2" + i;
c3.ID = "newC3" + i;
c4.ID = "newC4" + i;
c5.ID = "newC5" + i;
tr.ID = "newRow" + i;
c3.Controls.Add(ddl1);
c4.Controls.Add(ddl2);
c5.Controls.Add(t4);
tr.Cells.Add(c2);
tr.Cells.Add(c3);
tr.Cells.Add(c4);
tr.Cells.Add(c5);
Table4.Rows.Add(tr);
-----> this.Panel2.Controls.Add(Table4);
Session["table"] = Table4;
}
In line (this.Panel2.Controls.Add(Table4)), I'm getting
Object reference not set to an instance of an object-exception.
even though I have created a Panel in the design page I'm getting the error
Here any table is added dynamically not the Panel, Panel is created statically and I'm inserting dynamic table in it.

Answer 1

There are a couple of reasons this can happen, but the most important is:
Are you absolutely sure you have created a control with the exact id
with the runat attribute set to "server"?
This applies to both Panel2 and Table4.
Other than that you need to check these things:
Your class (where the exception is occuring) is inheriting from System.UI.Page
The #Page CodeBehind attribute is set to your class (spelling, capitalisation)
You're referencing the object at the correct time during the page lifecycle (i.e. before it's being initialised)
There's no where that the reference has been modified prior to use