"Collection was modified; enumeration operation may not execute." on form disposal

Question

"Collection was modified; enumeration operation may not execute." appears to be a common error with foreach loops, but I can't figure mine out. I have two classes of forms. One is begun on startup, and a button creates new instances of the second form, and displays them. When I close the secondary forms, I get an InvalidOperationException.

FirstForm.cs

public partial class FirstForm : Form
{
    SecondForm frmSecond;
    ...
    private void button1_Click(object sender, EventArgs e)
    {
        frmSecond= new SecondForm ();
        frmSecond.Show();
    }
}

SecondForm.designer.cs

partial class SecondForm
{
    ...
    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing); // InvalidOperationException thrown here.
    }
}

Answer 1

The problem was a PowerPacks.RectangleShape object I had placed on my second form and forgot about (because it wouldn't render.) I deleted the object, and the form disposed just fine.

Testing this further, I found that any control which requires a PowerPacks.ShapeContainer (LineShape, OvalShape, and RectangleShape,) cause this problem, but other PowerPacks objects don't.

I'm not sure why this happens, so if anyone figures out a workaround, I'd appreciate that. But for now I'll avoid PowerPacks shapes.