Nullpointer hibernate

@OneToOne(cascade=CascadeType.PERSIST, fetch=FetchType.LAZY, mappedBy ="data")
	@JoinColumn(name="ID")
	private HorasExtrasTO horasExtras;

@Id
	@GeneratedValue(generator="foreign")
    @GenericGenerator(name="foreign", strategy = "foreign", parameters={
      @Parameter(name="property", value="data")
    })
	@Column(name="ID")
	private Long id;

	@OneToOne(fetch=FetchType.LAZY, optional=true)
	@Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE})
	@PrimaryKeyJoinColumn
	private DataTrabalhadaTO data;