C# Windows form application InvalidCastException problem

This is a simple calculator program I am trying to make using Windows Forms Application in VS. The UnhandledException appears when I click anywhere except on the calculator buttons. I am fairly new to C# and it seems that a sender button in my function "common_operators" is causing the exception. Also, I want this calculator to have similar memory functionalities as windows 10's built-in calculator. I've searched everywhere but couldn't find a c# calculator implementation that is similar to Win10's built-in calculator has, I have already started but I think there's a better way to implement it. If u need more info, I've uploaded the "designer.cs" file that relates to the form application.

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


namespace WindowsFormsApp_Calculator
{ 
    
    public partial class CalculatorBase : Form
        {
            public double[] arrMemory = new double[5];
            public int indexer = 0;
            double result = 0;
            string asmd_operator = ""; // ASMD - Addition, Subtraction, Multiplication, Division
            bool insert_value = false;
            

        public CalculatorBase()
            {
                InitializeComponent();
            btn_mc.Enabled = false;
            btn_mr.Enabled = false;
            mem_textbox.Text = "There's nothing saved in memory";

            }

            private void Form1_Load(object sender, EventArgs e)
            {

            }

        private void numbers_zerotonine(object sender, EventArgs e)
        {
            Button b = (Button)sender;
            if((box_display.Text == "0") || insert_value)
                box_display.Text = "";
            insert_value = false;
            box_display.Text = box_display.Text + b.Text;
        }

        private void common_operators(object sender, EventArgs e)
        {
            Button b = (Button)sender;

            if (result != 0)
            {
                btn_eql.PerformClick();
                insert_value = true;
                asmd_operator = b.Text;
                subbox_display.Text = result + " " + asmd_operator;
            }
            else 
            {
                asmd_operator = b.Text;
                result = double.Parse(box_display.Text);
                box_display.Text = "";
                subbox_display.Text = System.Convert.ToString(result) + " " + asmd_operator;
            }
     
        }

        private void btn_ce_Click(object sender, EventArgs e)
        {
            box_display.Text = "0";
        }

        private void btn_c_Click(object sender, EventArgs e)
        {
            box_display.Text = "0";
            subbox_display.Text = "";
            result = 0;
        }

        private void btn_eql_Click(object sender, EventArgs e)
        {
            subbox_display.Text = "";
            switch(asmd_operator)
            {
                case "-":
                    box_display.Text = (result - double.Parse(box_display.Text)).ToString();
                    break;
                case "+":
                    box_display.Text = (result + double.Parse(box_display.Text)).ToString();
                    break;
                case "X":
                    box_display.Text = (result * double.Parse(box_display.Text)).ToString();
                    break;
                case "/":
                    box_display.Text = (result / double.Parse(box_display.Text)).ToString();
                    break;
                default:
                    break;
            }
            result = double.Parse(box_display.Text);
            asmd_operator = "";
        }

        private void btn_bs_Click(object sender, EventArgs e)
        {
            if(box_display.Text.Length > 0)
            {
                box_display.Text = box_display.Text.Remove(box_display.Text.Length - 1, 1);
            }
            if(box_display.Text == "")
            {
                box_display.Text = "0";
            }
        }

        private void btn_ms_Click(object sender, EventArgs e)
        {
            if (indexer == 5)
            {
                mem_textbox.Text = "Memory is full. Max limit = 5";
            }
            else
            {
                int hgt = 75;
                mem_textbox.Text = "";
                arrMemory[indexer] = double.Parse(box_display.Text);
                indexer++;
                btn_mc.Enabled = true;
                btn_mr.Enabled = true;

                TextBox mem = new TextBox();
                mem.Multiline = true;
                mem.TextAlign = HorizontalAlignment.Right;
                mem.Width = 275;
                mem.Height = 70;
                mem.Font = new Font(mem.Font.FontFamily, 20);
                mem.Text = box_display.Text;
                mem.Location = new Point(387, hgt); 
                this.Controls.Add(mem);
            }
        }

        private void btn_mc_Click(object sender, EventArgs e)
        {
            foreach (int i in arrMemory)
            {
                arrMemory[i] = 0;
            }
            indexer = 0;
            btn_mr.Enabled = false;
            btn_mc.Enabled = false;
            mem_textbox.Text = "There's nothing saved in memory";
        }

        private void btn_mr_Click(object sender, EventArgs e)
        {
            box_display.Text = arrMemory[indexer].ToString();
        }

        private void btn_mp_Click(object sender, EventArgs e)
        {
            arrMemory[indexer] += double.Parse(box_display.Text);
        }

        private void btn_mm_Click(object sender, EventArgs e)
        {
            arrMemory[indexer] -= double.Parse(box_display.Text);
        }


    }
}