An error occurs when calculating the sum of the matrix

Question

The user enters the dimension of the matrix and the program should calculate the sum of rows, columns and the total sum of the matrix. If the dimension of the matrix is ​​the same, for example, 2x2 3x3 7x7 then everything is fine. But if a 3x2 5x4 7x5 matrix, an exception occurs: An unhandled exception of type 'System.NullReferenceException' occurred in createElement_pr_4.exe

Additional information: The object reference does not indicate an object instance.

<Window x:Class="createElement_pr_4.MainWindow" 
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
xmlns:local="clr-namespace:createElement_pr_4" 
mc:Ignorable="d" 
Title="" Height="600" Width="1000">
    <Grid x:Name="Container">
        <TextBox HorizontalAlignment="Left" Height="23" TextWrapping="Wrap" Text="" VerticalAlignment="Top" Width="24" x:Name="HeightEl"/>
        <TextBox HorizontalAlignment="Left" Height="23" Margin="40,0,0,0" TextWrapping="Wrap" Text="" VerticalAlignment="Top" Width="28" x:Name="WidthEl"/>
        <Button Content="Создать таблицу" HorizontalAlignment="Left" Margin="92,0,0,0" VerticalAlignment="Top" Width="120" x:Name="Create" Click="Create_Click"/>
        <Button Content="Вычислить"  HorizontalAlignment="Left" Margin="61,178,0,0" VerticalAlignment="Top" Width="75" x:Name="Add" Click="Add_Click"/>
        <Label x:Name="test" Content="" HorizontalAlignment="Left" Margin="10,533,0,-7" VerticalAlignment="Top" Height="43" Width="120"/>
        <Label x:Name="row" Content="" HorizontalAlignment="Left" Margin="158,533,0,0" VerticalAlignment="Top" Width="377"/>
        <Label x:Name="col" Content="" HorizontalAlignment="Left" Margin="580,533,0,0" VerticalAlignment="Top" Width="377"/>
        <Label x:Name="label" Content="Общая сумма" HorizontalAlignment="Left" Margin="0,482,0,0" VerticalAlignment="Top"/>
        <Label x:Name="label1" Content="Сумма строк" HorizontalAlignment="Left" Margin="158,482,0,0" VerticalAlignment="Top"/>
        <Label x:Name="label2" Content="Сумма колонок" HorizontalAlignment="Left" Margin="580,482,0,0" VerticalAlignment="Top"/>





    </Grid>
</Window>

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
using System.Windows; 
using System.Windows.Controls; 
using System.Windows.Data; 
using System.Windows.Documents; 
using System.Windows.Input; 
using System.Windows.Media; 
using System.Windows.Media.Imaging; 
using System.Windows.Navigation; 
using System.Windows.Shapes; 

namespace createElement_pr_4
{
    public partial class MainWindow : Window
    {
        int w, h;
        TextBox[,] textBoxes = new TextBox[10, 10];
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Create_Click(object sender, RoutedEventArgs e)
        {
            foreach (var TextBoxItem in textBoxes)
            {
                Container.Children.Remove(TextBoxItem);
            }

            try
            {
                h = int.Parse(HeightEl.Text);
                if (h > 10)
                {
                    h = 0;
                    MessageBox.Show("error");
                }
                w = int.Parse(WidthEl.Text);
                if (w > 10)
                {
                    w = 0;
                    MessageBox.Show("error");
                }
            }
            catch (System.FormatException)
            {
                MessageBox.Show("error");
            }


            for (int i = 0; i < h; i++)
            {
                for (int j = 0; j < w; j++)
                {
                    TextBox textBox = new TextBox()
                    {
                        Height = 23,
                        Width = 23,
                        HorizontalAlignment = HorizontalAlignment.Left,
                        VerticalAlignment = VerticalAlignment.Top,
                        Margin = new Thickness(250.0 + (double)(j * 40), 100 + (double)(i * 40), 0, 0)
                    };
                    try
                    {
                        textBoxes[i, j] = textBox;
                        Container.Children.Add(textBoxes[i, j]);
                    }
                    catch (System.IndexOutOfRangeException)
                    {
                        MessageBox.Show("error");
                    }
                }
            }
            //Add.Visibility = Visibility.Visible;
            //Result.Visibility = Visibility.Visible;
        }

        private void Add_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                double sumCol;
                double sumRow;
                double sumAll = 0;

                for (int j = 0; j < w; j++)
                {
                    sumCol = 0;
                    sumRow = 0;
                    for (int i = 0; i < h; i++)
                    {
                        sumAll += Convert.ToDouble((textBoxes[i, j]).Text);
                        sumCol += Convert.ToDouble((textBoxes[i, j]).Text);
                        sumRow += Convert.ToDouble((textBoxes[j, i]).Text);
                    }
                    col.Content += Convert.ToString(sumCol + ", ");
                    row.Content += Convert.ToString(sumRow + ", ");
                }
                test.Content = sumAll;
            }
            catch (System.FormatException)
            {
                MessageBox.Show("erroe");
            }
            catch (System.OverflowException)
            {
                MessageBox.Show("error");
            }

        }
    }
}