Is this code to hook into a .NET window that we don't own safe? Can it be made safe?

Question

I have some code that is used to hook into third party code. It goes pretty much like below (mind you CodeWeDontHave can't be posted because we dont have the source). I have an issue where sometimes my code will deadlock on the line commented below. Also, we get a object reference error now and then.

Is this the most effective way to hook into a 3rd party program where we have no source code? Typically, in Winforms we aren't supposed to set controls in this way e.g. Control.Text = "blah"; from another Form, rather via properties & events. In fact, I'm surprised this works at all. Is it safe?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace BrianTests
{//this code requires a Form with a textbox called textBox1
    public partial class CodeWeDontHave : Form
    {
        [DllImport("user32.dll")]
        public extern static bool SetForegroundWindow(IntPtr hWnd);

        public CodeWeDontHave()
        {
            InitializeComponent();
        }

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            Text = "structured notes entry";

            //create another form that will attach to this one.
            Form formThatWeOwn = new Form();
            Button button = new Button() { Text = "upload text" };
            formThatWeOwn.Controls.Add(button);
            formThatWeOwn.Load += (a, b) => GetFormWithNoCode();
            button.Click += OtherFormButton_Click;
            formThatWeOwn.Show(this);
        }

        void OtherFormButton_Click(object sender, EventArgs e)
        {
            Button button = sender as Button;
            UploadRtfToNoCodeForm();
            //Close(); closes the parent...
            button.FindForm().Close();//hackjob            
        }

        private static Form GetFormWithNoCode()
        {
            Form noCodeForm = null;

            System.Threading.Thread.Sleep(50);

            noCodeForm = Application.OpenForms
                .Cast<Form>()
                .Where(form => form.Text.ToLower().StartsWith("structured notes entry"))
                .FirstOrDefault();

            return noCodeForm;
        }

        private static void UploadRtfToNoCodeForm()
        {
            Form noCodeForm = GetFormWithNoCode();

            var textBox = noCodeForm.Controls
                .Cast<Control>()
                .GetChildren(c => c.Controls.Cast<Control>())
                .Where(c => c is TextBox && String.Compare(c.Name, "textBox1", true) == 0)
                .LastOrDefault() as TextBox;

            string rtf = "here be some uploaded text";

            SetForegroundWindow(noCodeForm.Handle);//Does this handle need memory cleanup?

            noCodeForm.Focus();
            textBox.Focus();
            textBox.ReadOnly = false;//sometimes this line deadlocks...I can't reproduce in this test code
            textBox.Text = rtf;
            textBox.Update();
            textBox.Focus();
            textBox.ReadOnly = true;
        }
    }

    static class ExtensionStuff
    {
        public static IEnumerable<T> GetChildren<T>(this IEnumerable<T> startingParentSet, Func<T, IEnumerable<T>> childrenSelector, bool includeParent = true)
        {
            foreach (T item in startingParentSet)
            {
                foreach (T child in item.GetChildren(childrenSelector, includeParent))
                    yield return child;
            }
        }

        public static IEnumerable<T> GetChildren<T>(this T parent, Func<T, IEnumerable<T>> childrenSelector, bool includeParent = true)
        {
            if (parent == null) yield break;

            if (includeParent)
                yield return parent;

            IEnumerable<T> children = childrenSelector(parent);

            if (children == null) yield break;

            foreach (T child in children)
            {                
                foreach (T grandChild in child.GetChildren(childrenSelector, includeParent))
                    yield return grandChild;
            }

        }
    }
}