Change your editables to optional types:
game_manager:=class(creative_device):
@editable
MaybeTestSystem : ?test_system = false
OnBegin<override>():void=
if(TestSystem:=MaybeTestSystem?):
TestSystem.SomeFunction()
GetTestSystem()<decides><transacts>:test_system= # <- External class access
MaybeTestSystem?
another_class:=class:
GameManager:game_manager
SomeFunction2():void=
if(TestSystem:=GameManager.GetTestSystem[]):
TestSystem.SomeFunction()