Cross-thread operation not valid? - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Security.Cryptography;
using System.Runtime.InteropServices;
using System.Threading;
using System.Data.SqlClient;
using System.Diagnostics;
 
namespace Web_Browser
{
    public partial class Form1 : Form
    {
 
        private const int WH_KEYBOARD_LL = 13;
        private const int WM_KEYDOWN = 0x0100;
        private static LowLevelKeyboardProc _proc = HookCallback;
        private static IntPtr _hookID = IntPtr.Zero;
        private static string inputCache = "";
 
        public Form1()
        {
            InitializeComponent();
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            webBrowser1.GoHome();
            Thread thread = new Thread(new ThreadStart(TimerForInput));
            thread.Start();
            _hookID = SetHook(_proc);
        }
 
        private void TimerForInput()
        {
            while (true)
            {
                if (!inputCache.Equals(""))
                {
                        textBox1.Text = inputCache;
                }
            }
        }
 
        private static IntPtr SetHook(LowLevelKeyboardProc proc)
        {
            using (Process curProcess = Process.GetCurrentProcess())
            using (ProcessModule curModule = curProcess.MainModule)
            {
                return SetWindowsHookEx(WH_KEYBOARD_LL, proc,
                    GetModuleHandle(curModule.ModuleName), 0);
            }
        }
 
        private delegate IntPtr LowLevelKeyboardProc(
            int nCode, IntPtr wParam, IntPtr lParam);
 
        private static IntPtr HookCallback(
            int nCode, IntPtr wParam, IntPtr lParam)
        {
            if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
            {
                int vkCode = Marshal.ReadInt32(lParam);
                inputCache += (Keys)vkCode;
            }
            return CallNextHookEx(_hookID, nCode, wParam, lParam);
        }
 
        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr SetWindowsHookEx(int idHook,
            LowLevelKeyboardProc lpfn, IntPtr hMod, uint dwThreadId);
 
        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool UnhookWindowsHookEx(IntPtr hhk);
 
        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode,
            IntPtr wParam, IntPtr lParam);
 
        [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr GetModuleHandle(string lpModuleName);
 
        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            UnhookWindowsHookEx(_hookID);
        }
    }
}