I have an application in which I launch a window that displays byte data coming in from a 3rd party tool. I have included .CancelAsync() and .CancellationPending into my code (see below) but I have another issue that I am running into. private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e) { Thread popupwindow = new Thread(() => test()); popupwindow.Start(); // start test script if(backgroundWorker.CancellationPending == true) { e.Cancel = true; } } private voide window_FormClosing(object sender, FormClosingEventArgs e) { try { this.backgroundWorker.CancelAsync(); } catch (Exception ex) { MessageBox.Show(ex.Message.ToString()); } } Upon cancelling the test I get an `InvalidOperationException occurred" error from my rich text box in my pop-up window. It states that "Invoke or BeginInvoke" cannot be called on a control until the window handle has been created". I am not entirely sure what that means and would appreciate your help. LogWindow code for Rich Text Box: public void LogWindowText(LogMsgType msgtype, string msgIn) { rtbSerialNumberValue.Invoke(new EventHandler(delegate { rtbWindow.SelectedText = string.Empty; rtbWindow.SelectionFont = new Font(rtbWindow.SelectionFont, FontStyle.Bold); rtbWindow.SelectionColor = LogMsgTypeColor[(int)msgtype]; rtbWindow.AppendText(msgIn); rtbWindow.ScrollToCaret(); })); }