error: Value cannot be null. Parameter name: String

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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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        string s1, s2,oper;
        public Form1()
        {
            InitializeComponent();
        }

        private void btn0_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn0.Text;
        }

        private void btn1_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn1.Text;
        }

        private void btn2_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn2.Text;
        }

        private void btn3_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn3.Text;
        }

        private void btn4_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn4.Text;
        }

        private void btn5_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn5.Text;
        }

        private void btn6_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn6.Text;
        }

        private void BTn7_Click(object sender, EventArgs e)
        {
            textBox1.Text += BTn7.Text;
        }

        private void btn8_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn8.Text;
        }

        private void btn9_Click(object sender, EventArgs e)
        {
            textBox1.Text += btn9.Text;
        }

        private void btnpoint_Click(object sender, EventArgs e)
        {
            string s = textBox1.Text;
            int j = 0;
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] == '.')
                {
                    j = 0;
                    break;


                }
                else
                {
                    j = 1;
                }

             }
            if (j == 1)
            {
                textBox1.Text += btnpoint.Text;
            }
        }

        private void btnPlus_Click(object sender, EventArgs e)
        {
            string s1 = textBox1.Text;
            textBox1.Text = "";
            oper = "+";

        }

        private void btneql_Click(object sender, EventArgs e)
        {
            int e1;
            double equal;
            if (oper == "+")
            {
                string s = textBox1.Text;
                int j = 0;
                for (int i = 0; i < s.Length; i++)
                {
                    if (s[i] == '.')
                    {
                        j = 0;
                        break;


                    }
                    else
                    {
                        j = 1;
                    }

                }
                if (j == 1)
                {
                    string s2 = textBox1.Text;
                    e1 = int.Parse(s1) + int.Parse(s2);
                    textbox1.text=e1.ToString();
                }
                else
                {
                    string s2 = textBox1.Text;
                    equal = double.Parse(s1) + double.Parse(s2);
                     textbox1.text=equal.ToString();
                }
            }
        }
    }
}

In this calculator program when i try to add two number than error is coming "Value cannot be null. Parameter name: String"
please tell me how can i remove it??