Return Null after convert toObjects in Firestore

I have data class like below

data class Course (
    val title: String?=null,
    val image: String?=null,
    val guid: String?=null,
    val price: String?=null,
    val category: String?=null,
    val instructorName: String?=null,
    val instructorCode: String?=null,
    val rating: String?=null
)

and when i push data everything ok, i have my data in firestore like this(sorry i have 2 kind of data because try and error)

but my problem is when i use this execution in my viewmodel

val progressOne = async {
                firestore.collection("courses_popular")
                    .get()
                    .addOnCompleteListener {task ->

                        if(task.isSuccessful){
                            val doc = task.result
                            if(doc != null){
                                val res = doc.toObjects(Course::class.java)
                                popularData.postValue(Response(res, ResponseStatus.SUCCESS))
                            }else{
                                popularData.postValue(Response(responseStatus = ResponseStatus.ERROR, msg = "Document null"))
                            }
                        }
                        else{
                            popularData.postValue(Response(responseStatus = ResponseStatus.ERROR, msg = "${task.exception?.message}"))
                        }
                    }
            }

why i always got instructorName and instructorCode null, however the value are exists?
(this what i got when print console)

> MateriPopulerAdapter: Course(title=Belajar Matematika tingkat SMP,
> image=https://storage.googleapis.com/skill-baru.appspot.com/courses_popular/-MI7nXMm1EsCqQX2Shfr/ilustrasi-matematika.jpg,
> guid=1, price=500000, category=MTK, instructorName=null,
> instructorCode=null, rating=4)

1. ATTEMP
   change all camel case be normal case.

   data class Course (
   val title: String?=null,
   val image: String?=null,
   val guid: String?=null,
   val price: String?=null,
   val category: String?=null,
   val instructorname: String?=null,
   val instructorcode: String?=null,
   val rating: String?=null
   )

Error console

D/MateriPopulerAdapter: Course(title=Belajar Matematika tingkat SMP, image=https://storage.googleapis.com/skill-baru.appspot.com/courses_popular/-MI7nXMm1EsCqQX2Shfr/ilustrasi-matematika.jpg, guid=1, price=500000, category=MTK, instructorname=null, instructorcode=null, rating=4)
10-15 15:20:58.984 13736-13736/com.skillbaru.apps W/TabLayout: MODE_SCROLLABLE + GRAVITY_FILL is not supported, GRAVITY_START will be used instead
10-15 15:20:59.005 13736-13736/com.skillbaru.apps D/AndroidRuntime: Shutting down VM

I solved the issue, what i did is changing all parameters(in firestore and POJO) become lowercase (no camel case). And also remove all camelcase and try to uninstall the apps to ensure there is no caching params in local device. So now everything works fine.