Trouble with creating custom control in C#

I want to have a rounded custom button, for this aim I created a class extended from Button class:

    public class CustomButton : Button
    {
        //Fields
        private int borderSize = 0;
        private int borderRadius = 40;
        private Color borderColor = Color.PaleVioletRed;
        [Category("Custom Controls")]

        public int BorderSize
        {
            get
            {
                return borderSize;
            }
            set
            {
                borderSize = value;
                this.Invalidate();
            }
        }
        [Category("Custom Controls")]

        public int BorderRadius 
        { 
            get 
            {
                return borderRadius; 
            }
            set
            {
                borderRadius = value;
                this.Invalidate();
            }
        }
        [Category("Custom Controls")]
        public Color BorderColor { 
            get
            {
                return borderColor;
            }
            set
            {
                borderColor = value;
                this.Invalidate();
            }
        }
        [Category("Custom Controls")]
        public Color TextColor
        {
            get
            {
                return this.ForeColor;
            }
            set
            {
                this.ForeColor = value;
            }
        }
        //Constructor
        CustomButton()
        {
            this.FlatStyle = FlatStyle.Flat;
            this.FlatAppearance.BorderSize = 0;
            this.Size = new Size(150,40);
            this.BackColor = Color.MediumSlateBlue;
            this.ForeColor = Color.White;
        }
        //Methods
        private GraphicsPath GetFigurePath(RectangleF rect,float radius)
        {
            GraphicsPath path = new GraphicsPath();
            path.StartFigure();
            path.AddArc(rect.X, rect.Y, radius, radius, 180, 90);
            path.AddArc(rect.Width - radius, rect.Y, radius, radius, 270, 90);
            path.AddArc(rect.Width - radius, rect.Height - radius, radius, radius, 0, 90);
            path.AddArc(rect.X, rect.Height-radius, radius, radius, 90, 90);
            path.CloseFigure();
            return path;
        }
        protected override void OnPaint(PaintEventArgs pevent)
        {
            base.OnPaint(pevent);
            pevent.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            RectangleF rectSurface = new RectangleF(0, 0, this.Width, this.Height);
            RectangleF rectBorder = new RectangleF(1, 1, this.Width - 0.8F, this.Height - 1);

            if(borderRadius > 2) //Rounded Button
            {
                using (GraphicsPath pathSurface = GetFigurePath(rectSurface, borderRadius))
                using (GraphicsPath pathBorder = GetFigurePath(rectBorder, borderRadius-1F))
                using (Pen penSurface = new Pen(this.Parent.BackColor, 2))
                using (Pen penBorder = new Pen(borderColor, borderSize))
                {
                    penBorder.Alignment = PenAlignment.Inset;
                    //button Surface
                    this.Region = new Region(pathSurface);

                    //Draw Surface Border for HD result
                    pevent.Graphics.DrawPath(penBorder, pathBorder);

                    //Button border
                    if (borderSize >= 1)
                        //Draw Control border
                        pevent.Graphics.DrawPath(penBorder, pathBorder);
                }
            }
            else //Normal Button
            {
                //Button surface
                this.Region = new Region(rectSurface);
                //Button border
                if(borderSize >= 1)
                {
                    using (Pen penBorder = new Pen(borderColor, borderSize))
                    {
                        penBorder.Alignment = PenAlignment.Inset;
                        pevent.Graphics.DrawRectangle(penBorder, 0, 0, this.Width - 1, this.Height - 1);
                    }
                }
            }
        }
        protected override void OnHandleCreated(EventArgs e)
        {
            base.OnHandleCreated(e);base.OnHandleCreated(e);
            this.Parent.BackColorChanged += new EventHandler(Container_BackColorChanged);
        }

        private void Container_BackColorChanged(object sender, EventArgs e)
        {
            if (this.DesignMode)
                this.Invalidate();
        }

        private void InitializeComponent()
        {
            this.SuspendLayout();
            this.ResumeLayout(false);

        }
    }
}

But when I use my custom button, I get this warning:

Field 'frmMainPanel.customButton1' is never assigned to, and will always have its default value null

And when running app, get: in this error I get 'Object reference not set to an instance of an object.' but can't resolve this problem