No accessors or field is found for property var XXXXXX: kotlin.String?

[freelancerweb1991]

model= {
var id: Int? = null
lateinit var name: String
lateinit var localPeerId: String
var remotePeerId: String? = null
@JSON(serializeNull = false)
var walletAddress: String? = null
@JSON(serializeNull = false)
var avatar: String? = null
var isInitialized: Boolean = false
var initializeDate: ZonedDateTime? = null
var createDate: ZonedDateTime? = null
}
val queryResult = dataProvider.getData() as MutableList
val queryResultJson = JsonArray(queryResult)
var result = queryResultJson.toJsonString()

In the above code sample after installing app-release.apk has null refrence error for fields but in debug on emulator no error.
I used implementation "com.beust:klaxon:5.6" in buld.gradle.

in line "var result = queryResultJson.toJsonString()"

we have this error:
No accessors or field is found for property var ea.c.avatar: kotlin.String?

b7.a0: No accessors or field is found for property var ea.c.avatar: kotlin.String?
at b7.w.b(Unknown Source:259)
at b7.w.a(Unknown Source:0)
at b7.v$c$a.a(Unknown Source:3)
at b7.v$c$a.invoke(Unknown Source:0)
at b7.c0$b.invoke(Unknown Source:11)
at b7.c0$c.b(Unknown Source:0)
at b7.v$c.t(Unknown Source:7)
at b7.f.call(Unknown Source:5)
at q0.e.a(Unknown Source:310)
at q0.w.e(Unknown Source:220)
at q0.i.h(Unknown Source:51)
at q0.k$a.a(Unknown Source:11)
at q0.i.b(Unknown Source:0)
at q0.k$a.b(Unknown Source:10)
at q0.i.a(Unknown Source:0)
at q0.k$a.c(Unknown Source:13)
at org.example.app.MainActivity.U(Unknown Source:1517)
at org.example.app.MainActivity.T(Unknown Source:15)
at org.example.app.MainActivity.L(Unknown Source:0)
at da.d.f(Unknown Source:2)
at a6.j$a.a(Unknown Source:17)
at n5.c.j(Unknown Source:18)
at n5.c.k(Unknown Source:20)
at n5.c.g(Unknown Source:0)
at n5.b.run(Unknown Source:12)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:246)
at android.app.ActivityThread.main(ActivityThread.java:8633)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)