MasterDetailPage can not back

Hi all
I create a MasterDetailPage and insert a button on Master. When I click on button go to MySosRequests.
But in MySosRequestswhen I click on back of android app close. There is no back arrow on top left of MySosRequestsfor backing.
Whats wrong?

<MasterDetailPage.Master>
    <ContentPage Title="منو">
        <StackLayout Orientation="Vertical">
            <Button HorizontalOptions="Center" Text="درخواست های من" Command="{Binding GoToMySosRequests}" />
        </StackLayout>
    </ContentPage>
</MasterDetailPage.Master>

<MasterDetailPage.Detail Title="">
    <NavigationPage>
        <x:Arguments>
            <pages:MainView />
        </x:Arguments>
    </NavigationPage>
</MasterDetailPage.Detail>

using Bit.ViewModel;
using Prism.Navigation;

namespace Sanaap.App.ViewModels
{
public class MenuViewModel : BitViewModelBase
{
public BitDelegateCommand GoToMySosRequests { get; set; }

    public MenuViewModel(INavigationService navigationService)
    {
        GoToMySosRequests = new BitDelegateCommand(async () =>
        {
            await navigationService.NavigateAsync("MySosRequests");
        });
    }
}

}