i have a xamarin.form project and try to add unit test, nut having an issue which made me crazy,
this is my viewmodel

 public class ClientsViewModel : XamarinViewModel
 {
     private readonly IClientAppService _clientAppService;
...
}

in my test class when I reach to second line it throw null refrence error,

  _mockClientAppService = new Mock<IClientAppService>();
    var viewModel = new ClientsViewModel(_mockClientAppService.Object);

this is my XamarinViewModel constructor

protected XamarinViewModel()
 {
     NavigationService = DependencyResolver.Resolve<INavigationService>();
     LoginInfoSaveService = DependencyResolver.Resolve<ILoginInfoSaveService>();
     ObjectMapper = NullObjectMapper.Instance;
 }

the exact error happens on DependencyResolver.Resolve, which throw null refrenc because ApplicationBootstrapper is null

  public static  class DependencyResolver
  {
      public static IIocManager IocManager => ApplicationBootstrapper.AbpBootstrapper.IocManager;

      public static T Resolve<T>()
      {
          return ApplicationBootstrapper.AbpBootstrapper.IocManager.Resolve<T>();
      }
    ...
}

Message: 
Initialization method WGMobileUnitTest.Client.ClientsViewModelTests.Setup threw exception. System.NullReferenceException: Object reference not set to an instance of an object..

Stack Trace: 
DependencyResolver.ResolveT line 12
ClientsViewModelTests.Setup() line 43