How to solve exception of NullReferenceException?

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C#

C#4.0

I am trying to write code for multiple client server and it is working . But I want to display data conversation on form textbox, for that I have following method in form1.cs which if calling from same class this runs but when calling from another class handleClient.cs this doesnt works and giving "object reference not set to an instance : null reference exception" error but when I putted breakpoint there it shows data in loop but break just after the loop and doesn't displays data.
The error is at the bold line in "AppendTxtdata" method.

here is the code for better understanding,

form1.cs.

public void AppendTxtdata(string val)
{
try
{
if (InvokeRequired)
{
this.Invoke(new Action<string>(AppendTxtdata), new object[] { val });
return;
}
txtdata.Text += val;
}
catch { }
}

handleClient.cs

public void GetData() // to run server //
{
Form1 f = new Form1();

try
{
NetworkStream networkStream = clientSocket.GetStream();


for (int p = 0; p >= 0; p++)
{
byte[] b = new byte[100];

int k = networkStream.Read(b, 0, b.Length);

f.AppendTxtdata("The Data from Client ( " + clNo + " ) Recieved..." + Environment.NewLine);

for (int i = 0; i < k; i++)
{
f.AppendTxtdata("" + Convert.ToChar(b[i]));
}

f.AppendTxtdata(Environment.NewLine);

var j = b.Length - 1;
while (b[j] == 0)
{
    --j;
}

    var temp = new byte[j + 1];
    Array.Copy(b, temp, j + 1);

    string res = System.Text.Encoding.ASCII.GetString(temp);

}