We have the same issue trying to work with Control Rig in the 5.0 Preview 1. On the CPP side of things, the AddMappedSkeletalMesh function is now marked as ‘UnsafeDuringActorConstruction’ which means it cannot be called in the construction script.
We tried to use the function during the PostInitialize event, but the error message provided in the function UControlRigComponent::EnsureCalledOutsideOfBracket says that the AddMappedSkeletalMesh function should be called during the construction script.
It seems like ControlRig has a new intended use for this function, but it seems like were gonna have to wait for some guidance/documentation to know what that is.