OK, what dumb mistake am I making?

I look at the quickwatch window, and ft (form) is *not* null.

if (ft == null)
{
ft = new FormTypes(this);
}
ft.Show();

I get the error "Value cannot be null." on ft.Show().

I'm stumped. It must be referring to something else, but this is a button.click() event nothing else but this code. I have ft defined private in the form.

Related question:

I use this same logic to open some other forms and it works, but only the first time. The second time it doesn't do the new but I get a different type of error - something about object having been disposed. So:

1) Why the null error on this example.
2) Why in similar situations is it *not* null, but gives an error that it was disposed. You mean, when it disposes it, it doesn't make it null? How the heck am I supposed to know if I can do a new again?