Custom Control Unhandled Exception in the Designer

I'm using code for a custom tab control that I found online and modified to suit my needs.

protected override void OnPaint(PaintEventArgs e) {
    this.PaintTransparentBackground(e.Graphics, this.ClientRectangle);
    this.PaintAllTabs(e);

    if (this.TabCount > 0 && ShowUnderline)
        e.Graphics.DrawLine(new Pen(Colors.TAB_SELECTED, 1), UnderlineStartOffset, 23, this.ClientRectangle.Width - UnderlineEndOffset, 23);
}

protected void PaintAllTabs(PaintEventArgs e) {
    if (this.TabCount > 0)
    {
        CursorSet = false;

        for (int index = this.TabCount - 1; index > -1; index--)
        {
            this.PaintTab(e, index);
            this.PaintTabText(e, index);
            this.TabPages[index].BackColor = Colors.CUSTOM_BLACK_2;
        }
    }
}

I also added some properties to be editable in the designer (not sure if this is how it is meant to be done):

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Tab X Offset")]
    public int XOffset { get; set; }

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Tab Y Offset")]
    public int YOffset { get; set; }

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Tab Width Offset")]
    public int WOffset { get; set; }

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Show Tab Underline")]
    public bool ShowUnderline { get; set; }

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Underline Start Offset")]
    public int UnderlineStartOffset { get; set; }

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Underline End Offset")]
    public int UnderlineEndOffset { get; set; }

    [Browsable(true)]
    [Category("Customizations")]
    [DisplayName("Enable Tab Closing")]
    public bool EnableTabClosing { get; set; }

The result is it looks great in the designer and at run-time, providing the Designer is not selected when running. If it is, I get the following error:

The control App.Custom_Controls.CustomTabControl has thrown an unhandled exception in the designer and has been disabled.

Exception: The file ..\View_Main.cs cannot be modifed in the designer while building or debugging.

If I surround the OnPaint with

if(!this.DesignMode) { }

The error is resolved but I lose the painting of my tab control in the designer.