Skeletal mesh defined in code cannot be set in the editor.

Defining a skeletal mesh component in code and attaching it to the root component will not allow the component's details to be editable inside the editor.

1. Open UE4Editor (any project)
2. Add code to project based on actor (MyActor)
3. In the header file add the following:

UPROPERTY()
USkeletalMeshComponent* Skeletal_Mesh;

4. In the source file add the following to the constructor:

Skeletal_Mesh = CreateDefaultSubobject<USkeletalMeshComponent>(TEXT("Skeletal_Mesh"));
Skeletal_Mesh->AttachTo(RootComponent).

5. Compile and create a blueprint basted MyActor

Results:
When the blueprint is placed in the level and the skeletal mesh component is selected the details panel is entirely gray'ed out. When the blueprint is opened the details panel is completely blank.