Like FreedTerror mentioned, the reason for it is in fact the assembly definition.
There are 2 ways you can fix this:
Use the inherited class instead
There is a class called "DefaultChallengeModeGUI". This class inherits ChallengeMode and its outside the assembly scope. You can access anything from the parent class as well as make references to your singleton.
Delete the assembly definitions (Source version)
Delete the following files:
\UFE\Engine\Scripts\Core\UFE3D.asmdef
\UFE\Engine\Editor\UFE3D.Editor.asmdef
These definitions are not needed in the Source version. Deleting these files will "free" the core scripts from enclosure and you will be able to make your static reference from there.