Form1.cs

    public string tags;
    public List<string> taglist;

    private void button1_Click(object sender, EventArgs e)
    {
        openFileDialog1.Filter = "TXT (*.txt)|*.txt|ALL (*.*)|*.*";

        if (openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
        {
            //Reads file like fuckin ghetto and remove empty lines
            label1.Text = openFileDialog1.FileName;
            textBox1.Text = File.ReadAllText(label1.Text);
            textBox2.Text = Regex.Replace(textBox1.Text, @"^s+$[rn]*", "", RegexOptions.Multiline);

            //Save file which empty lines removed
            File.WriteAllText(label1.Text +"2", textBox2.Text);

            //Read file which empty lines removed
            string readerline;
            StreamReader reader = new StreamReader(label1.Text + "2");

            //While loop lines untill add all lines to list
            while((readerline = reader.ReadLine()) != null)
            {
                taglist.Add(readerline);
                textBox3.AppendText ("Readerline Text:" + (readerline) + "rn");
            }

            tags = textBox2.Text;

            //remove temp txtfile
            reader.Dispose();
            File.Delete(label1.Text + "2");
            Program.URLadder(taglist); // Fucking error appears at here
        }
    }

    public void MainDbgWriter(string text)
    {
        if (this.InvokeRequired)
        {
            this.textBox3.Text += text;
        }

    }

    Program.cs

    public void URLadder(List <string> taglist)
    {
        foreach(string tag in taglist)
        {
            var fixedtags = prefix + tag + suffix;
            var fixedtags2 = prefix2 + tag + suffix;
            var fixedtags3 = prefix3 + tag + suffix;
            ResponseCheck(fixedtags);

        }

    }

    public void MainDbgWriter(string text)
    {
        fm.textBox3.AppendText(text + Environment.NewLine);
    }