NullPointerException when using PersistentEntityResourceAssembler.getResource on existing Obj instance

Question

I'm trying to build a restful controller with HAL links and custom methods. When I'm using a getResources from PersistentEntityResourceAssembler (HAL links) and this cause NullPointer.

StackTrace

java.lang.NullPointerException: null
    at pl.com.tt.sok.core.component.attendance.entity.PersonAttendanceListBuilder.addAttendance(PersonAttendanceListBuilder.java:37)
    at pl.com.tt.sok.core.component.attendance.service.AttendanceStatusService.createAttendanceListForPersonBasedOnRequestedDates(AttendanceStatusService.java:50)
    at pl.com.tt.sok.core.controller.attendance.AttendanceStatusController.getPersonAttendanceList(AttendanceStatusController.java:34)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497) ...

PersonAttendanceListBuilder

@Autowired
private PersistentEntityResourceAssembler assembler;
(...)

private PersistentEntityResourceAssembler assembler;
    public PersonAttendanceListBuilder addAttendance(Attendance attendance) {
    PersistentEntityResource resource = assembler.toResource(attendance); //(37)
    this.attendanceList.add(resource);
    return this;
}

AttendanceStatusService

            attendance = attendanceRepository.findByDateAndPerson(Date.valueOf(day), person);
        if(attendance==null){
            attendance = new Attendance();
       }
        listBuilder.addAttendance(attendance); //(50)