Java.Lang.NullPointerException

toda vez que eu clico no botão tirar foto aparece a mensagem de erro

Java.Lang.NullPointerException

[assembly:Xamarin.Forms.Dependency(typeof(MainActivity))]
namespace TesteDrive.Droid
{
    [Activity(Label = "TesteDrive", Icon = "@drawable/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
                ,ICamera
    {//----------------------------------------------------------
        static Java.IO.File arquivoImagem;
        public void TirarFoto()
        {
            var intent = new Intent(MediaStore.ActionImageCapture);

            intent.PutExtra(MediaStore.ExtraOutput, Android.Net.Uri.FromFile(arquivoImagem));

            var activity = Forms.Context as Activity;
            activity.StartActivityForResult(intent, 0);
        }

        private static Java.IO.File PegarArquivoImagem()
        {
            Java.IO.File arquivoImagem;
            Java.IO.File diretorio = new Java.IO.File(
                Android.OS.Environment.GetExternalStoragePublicDirectory(
                    Android.OS.Environment.DirectoryPictures), "Images");

            arquivoImagem = PegarArquivoImagem();

            if (!diretorio.Exists())
                diretorio.Mkdirs();


            arquivoImagem = new Java.IO.File(diretorio, "MinhaFoto.jpg");
            return arquivoImagem;
        }

        //-------------------------------------------------------
        protected override void OnCreate(Bundle bundle)
        {
            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

            base.OnCreate(bundle);

            global::Xamarin.Forms.Forms.Init(this, bundle);
            LoadApplication(new App());
        }
        //--------------------------------------------------------------------------------------------
        protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
        {
            base.OnActivityResult(requestCode, resultCode, data);

            if(resultCode == Result.Ok)
            {
                byte[] bytes;

                using(var stream = new Java.IO.FileInputStream(arquivoImagem))
                {
                    bytes = new byte[arquivoImagem.Length()];
                    stream.Read(bytes);
                }

                MessagingCenter.Send<byte[]>(bytes, "FotoTirada");
            }
        }
    }
}

namespace TesteDrive.ViewModels
{
    public class MasterViewModel : BaseViewModel
    {

        //-----------------------------------------------
        public ImageSource fotoPerfil ="Perfil.png";

        public ImageSource FotoPerfil
        {
            get { return fotoPerfil; }
            private set{ fotoPerfil = value;  OnPropertyChanged(); }
        }
        //----------------------------------------------

        private readonly Usuario usuario;

        public ICommand EditarPerfilCommand { get; private set; }
        public ICommand SalvarCommand { get; private set; }
        public ICommand EditarCommand { get; private set; }
        public ICommand TirarFotoCommand { get; private set; }

            MessagingCenter.Subscribe<byte[]>(this,"FotoTirada",
                (bytes)=>
                {
                    FotoPerfil = ImageSource.FromStream(
                        () => new MemoryStream(bytes));
                });
        }
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    xmlns:converters="clr-namespace:TesteDrive.Converters"
    x:Class="TesteDrive.Views.MasterView"
    Title="Perfil">
    <TabbedPage.Resources>
        <ResourceDictionary>
            <converters:NegativoConverter x:Key="negativo"></converters:NegativoConverter>
        </ResourceDictionary>
    </TabbedPage.Resources>
    <TabbedPage.Children>
        <ContentPage Title="USUÁRIO">
            <Frame OutlineColor="Silver"
                Margin="15"
                VerticalOptions="CenterAndExpand">
                <StackLayout HorizontalOptions="Center" VerticalOptions="Center">
                    <Image Source="{Binding FotoPerfil}" HeightRequest="100" WidthRequest="100"></Image>
                    <Label Text="{Binding Nome}"></Label>
                    <BoxView HeightRequest="1" HorizontalOptions="Fill" Color="Gray"></BoxView>
                    <Label Text="{Binding Email}"></Label>
                    <BoxView HeightRequest="1" HorizontalOptions="Fill" Color="Gray"></BoxView>
                    <Button Text="PERFIL" Command="{Binding EditarPerfilCommand}"></Button>
                </StackLayout>
            </Frame>
        </ContentPage>
        <ContentPage Title="EDITAR">
            <Frame OutlineColor="Silver"
                    Margin="15"
                    VerticalOptions="CenterAndExpand">
                    <StackLayout HorizontalOptions="Center" VerticalOptions="Center">
                    <Image Source="{Binding FotoPerfil}" HeightRequest="100" WidthRequest="100"></Image>
                    <Button Text="Tirar Foto" Command="{Binding TirarFotoCommand}"></Button>
                        <TableView>
                            <TableRoot>
                                <TableSection Title="Dados Pessoais">
                                    <EntryCell Placeholder="Nome" Text="{Binding Nome}" IsEnabled="{Binding Editando}"></EntryCell>
                                    <EntryCell Placeholder="Data de Nascimento" Text="{Binding DataNascimento}" IsEnabled="{Binding Editando}"></EntryCell>
                                    <EntryCell Placeholder="Telefone" Text="{Binding Telefone}" IsEnabled="{Binding Editando}"></EntryCell>
                                    <EntryCell Placeholder="Email" Text="{Binding Email}" IsEnabled="{Binding Editando}"></EntryCell>
                                </TableSection>
                            </TableRoot>
                        </TableView>
                        <Button Text="SALVAR" Command="{Binding SalvarCommand}" IsVisible="{Binding Editando}"></Button>
                        <Button Text="EDITAR" Command="{Binding EditarCommand}" IsVisible="{Binding Editando, Converter={StaticResource Key=negativo}}"></Button>
                    </StackLayout>
                </Frame>
        </ContentPage>
    </TabbedPage.Children>
</TabbedPage>