当我将文本框保留为空时如何防止程序崩溃c＃

using System;    
using System.Collections.Generic;    
using System.ComponentModel;    
using System.Data;    
using System.Drawing;    
using System.Linq;    
using System.Text;    
using System.Windows.Forms;

namespace IP_HULK
{

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            int n1, n2, n3, n4;
            if (numbox1.Text == "" || numbox2.Text == "" || numbox3.Text == "" || numbox4.Text == "")
            {
                MessageBox.Show("el ip fadi");
            }
            n1 = Convert.ToInt16(numbox1.Text);
            n2 = Convert.ToInt16(numbox2.Text);
            n3 = Convert.ToInt16(numbox3.Text);
            n4 = Convert.ToInt16(numbox4.Text);
            if ((n1 > 0 && n1 < 255) && (n2 > -1 && n2 < 255) && (n3 > -1 && n3 < 255) && (n4 > -1 && n4 < 255))
                            MessageBox.Show("The IP is Valid");
                        else
                            MessageBox.Show("The ip isn't valid");
        }

        private void textBox1_TextChanged_1(object sender, EventArgs e)
        {

        }

        private void numbox1_Leave(object sender, EventArgs e)
        {

        }
    }
}