ensure fires when accessing USlateScreenReaderEngineSubsystem as a dependency

Steps to Reproduce
Create a `UEngineSubsystem` in a game module that accesses `USlateScreenReaderEngineSubsystem` as a dependency in `Initialize()`.

For example:

`UCLASS()
class NEBULA_API UMySubsystem : public UEngineSubsystem
{
GENERATED_BODY()

virtual void Initialize(FSubsystemCollectionBase& Collection) override
{
Super::Initialize(Collection);

// This will cause an ensure to fire
USlateScreenReaderEngineSubsystem* ScreenReaderSubsystem = Collection.InitializeDependency();
}
}`