new Geocoder()).GetPositionsForAddressAsync error for android

I got this error, I don't know how to fix

[MonoDroid] UNHANDLED EXCEPTION: Java.Lang.NullPointerException: Exception of type 'Java.Lang.NullPointerException' was thrown.
[MonoDroid] at Android.Runtime.JNIEnv.CallNonvirtualVoidMethod (IntPtr jobject, IntPtr jclass, IntPtr jmethod, Android.Runtime.JValue[] parms) [0x00000] in :0
[MonoDroid] at Android.Runtime.JNIEnv.FinishCreateInstance (IntPtr instance, IntPtr jclass, IntPtr constructorId, Android.Runtime.JValue[] constructorParameters) [0x00000] in :0
[MonoDroid] at Android.Locations.Geocoder..ctor (Android.Content.Context context) [0x00000] in :0
[MonoDroid] at Xamarin.Forms.Maps.Android.GeocoderBackend+d__2.MoveNext () [0x00000] in :0
[MonoDroid] --- End of stack trace from previous location where exception was thrown ---
[MonoDroid] at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x00000] in :0
[MonoDroid] at System.Runtime.CompilerServices.TaskAwaiter1[System.Collections.Generic.IEnumerable1[Xamarin.Forms.Maps.Position]].GetResult () [0x00000] in :0
[MonoDroid] at DAI.POC.DealerMap+c__async0.MoveNext () [0x00000] in :0
[MonoDroid] --- End of managed exception stack trace ---
[MonoDroid] java.lang.NullPointerException
[MonoDroid] at android.location.GeocoderParams.(GeocoderParams.java:50)
[MonoDroid] at android.location.Geocoder.(Geocoder.java:83)
[MonoDroid] at android.location.Geocoder.(Geocoder.java:95)
[MonoDroid] at dai.poc.android.MainActivity.n_onCreate(Native Method)
[MonoDroid] at dai.poc.android.MainActivity.onCreate(MainActivity.java:28)
[MonoDroid] at android.app.Activity.performCreate(Activity.java:5231)
[MonoDroid] at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
[MonoDroid] at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2148)
[MonoDroid] at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
[MonoDroid] at android.app.ActivityThread.access$800(ActivityThread.java:135)
[MonoDroid] at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
[MonoDroid] at android.os.Handler.dispatchMessage(Handler.java:102)
[MonoDroid] at android.os.Looper.loop(Looper.java:136)
[MonoDroid] at android.app.ActivityThread.main(ActivityThread.java:5001)
[MonoDroid] at java.lang.reflect.Method.invokeNative(Native Method)
[MonoDroid] at java.lang.reflect.Method.invoke(Method.java:515)
[MonoDroid] at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
[MonoDroid] at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
[MonoDroid] at dalvik.system.NativeStart.main(Native Method)
[mono]
[mono] Unhandled Exception:
[mono] Java.Lang.NullPointerException: Exception of type 'Java.Lang.NullPointerException' was thrown.
[mono] at Android.Runtime.JNIEnv.CallNonvirtualVoidMethod (IntPtr jobject, IntPtr jclass, IntPtr jmethod, Android.Runtime.JValue[] parms) [0x00000] in :0
[mono] at Android.Runtime.JNIEnv.FinishCreateInstance (IntPtr instance, IntPtr jclass, IntPtr constructorId, Android.Runtime.JValue[] constructorParameters) [0x00000] in :0
[mono] at Android.Locations.Geocoder..ctor (Android.Content.Context context) [0x00000] in :0
[mono] at Xamarin.Forms.Maps.Android.GeocoderBackend+d__2.MoveNext () [0x00000] in :0
[mono] --- End of stack trace from previous location where exception was thrown ---
[mono] at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x00000] in :0
[mono] at System.Runtime.CompilerServices.TaskAwaiter1[System.Collections.Generic.IEnumerable1[Xamarin.Forms.Maps.Position]].GetResult ()
[mono-rt] [ERROR] FATAL UNHANDLED EXCEPTION: Java.Lang.NullPointerException: Exception of type 'Java.Lang.NullPointerException' was thrown.
[mono-rt] at Android.Runtime.JNIEnv.CallNonvirtualVoidMethod (IntPtr jobject, IntPtr jclass, IntPtr jmethod, Android.Runtime.JValue[] parms) [0x00000] in :0
[mono-rt] at Android.Runtime.JNIEnv.FinishCreateInstance (IntPtr instance, IntPtr jclass, IntPtr constructorId, Android.Runtime.JValue[] constructorParameters) [0x00000] in :0
[mono-rt] at Android.Locations.Geocoder..ctor (Android.Content.Context context) [0x00000] in :0
[mono-rt] at Xamarin.Forms.Maps.Android.GeocoderBackend+d__2.MoveNext () [0x00000] in :0
[mono-rt] --- End of stack trace from previous location where exception was thrown ---
[mono-rt] at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x00000] in :0
[mono-rt] at System.Runtime.CompilerServices.TaskAwaiter1[System.Collections.Generic.IEnumerable1[Xamarin.Forms.Maps.Position]]

any suggestions?