When you close the form, it gets disposed. The variable is still pointing to the object, you just can't use it since it is marked for gc (something like that, close enough right?)
Try this:

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

>The .show() giving a null error message has mysteriously quit happening.
>
>But, I still don't know why I'm getting the dispose error in the other cases