This is one of those “D’oh!” moments.  You’re creating your own UITypeEditor.  You know the UITypeEditor class is located in the System.Drawing.Design namespace.  So naturally you want to add to your Visual Studio project a reference to the System.Drawing.Design.dll, right?  Wrong!  When you compile your project, the following error may appear:

The type or namespace name ‘UITypeEditor’ could not be found (are you missing a using directive or an assembly reference?)

It turns out that UITypeEditor is actually defined in System.Drawing.dll, even though it’s located in the System.Drawing.Design namespace.  See the disconnect?  But you can easily solve this problem by adding to your Visual Studio project a reference to System.Drawing.dll.