Ok well, I seem to have rectified the problem.
I changed them from structs to classes, and split the lists into a private and public in order to give them a default value. I also created some constructors for both. Now the designer seems to have no trouble at all.

eg:
Code:
            private List<ToolboxItem> items = new List<ToolboxItem>();
            public List<ToolboxItem> Items { get { return items; } set { items = value; } }