How do I read json data (Latitude, Longitude) and convert it to polyline (Kotlin)

I have a list of lat,long data in json format (AIRPORTS.jsin), I want to read it and convert it to polylines. After writing a function to do that, It does not show in the mapActivity

This is what I have done

fun getPolylines(fileName: String?): 
    MutableList<LatLng>? {
        var polyline: LatLng

        // array list of lines init
        val polylines = mutableListOf<LatLng>()
        val inputStream = fileName?.let { 
            application.assets.open(it).bufferedReader() 
        }
        var Json_string = inputStream.use {
            it?.readText() ?: null
        }
        // get json array
        val jsonArray = JSONArray(Json_string)
        for (i in 0 until jsonArray.length()) {
            var jsonPolyline: JSONObject
            try {
                jsonPolyline = jsonArray.getJSONObject(i)
                polyline = LatLng(
                    (jsonPolyline.getDouble("lat")),
                    (jsonPolyline.getDouble("lon"))
                )
                polylines.add(polyline)
            } catch (e: JSONException) {
                Log.d(ContentValues.TAG, "JSONException reading polylines: " + e.message)
            } catch (e: Exception) {
                Log.d(ContentValues.TAG, "Exception reading polylines: " + e.message)
            }

        }
        return polylines

    }

I then called it in onMapListener

//add polylines
var polylines: MutableList<LatLng>? = getPolylines("IPO_OMAGWA.json")
map.addPolyline(PolylineOptions()
    .addAll(polylines!!)
    .pattern(Arrays.asList(Dot(), Gap(10.0f)))
    .geodesic(true)
    .zIndex(5F)
)

but its not showing