我试图在注销按钮点击时设置MainPage，但我得到的是下面提到的异常。
退出功能
private void Logout()
{
//Settings.Current.Remove(Constants.PREF_IS_LOGGED_IN);
//Settings.Current.Remove(Constants.PREF_AUTH_TOKEN);
MainPage = new NavigationPage(new LoginPage());
}
LoginPageViewModel：
using Prism.Mvvm;
using Prism.Navigation;
using SalesApp.Controllers.RestController;
using System;
using System.Windows.Input;
using Xamarin.Forms;
using Acr.Settings;
using Plugin.Connectivity;
using SalesApp.Utils;
using SalesApp.Views;
using SalesApp.Views.SideMenu;
using Acr.UserDialogs;
using SalesApp.Interfaces;
namespace SalesApp.ViewModels
{
public class LoginPageViewModel : BindableBase, INavigationAware
{
private String _email = Constants.Username; // dummy value
private String _password = Constants.Password; // dummy value
private Boolean _isShowPassword = true;
private Boolean _isShowPasswordIcon = false;
private Boolean _isRememberMe = false;
ICommand _tapCommandShowPassword;
ICommand _tapCommandNavigateForgotPassword;
ICommand _login;
INavigationService _navigationService;
public DataManager DataManager { get; private set; }
public LoginPageViewModel(INavigationService navigationService)
{
_tapCommandShowPassword = new Command(ShowHidePassword);
_tapCommandNavigateForgotPassword = new Command(NavigateToForgotPassword);
_login = new Command(LoginApiCalls);
_navigationService = navigationService;
}
public String Email
{
get
{
return _email;
}
set
{
_email = value;
}
}
public String Password
{
get
{
return _password;
}
set
{
_password = value;
if (_password.Length > 0)
IsShowPasswordIcon = true;
else
{
IsShowPasswordIcon = false;
IsShowPassword = true;
}
}
}
public bool IsShowPassword
{
get
{
return _isShowPassword;
}
set
{
_isShowPassword = value;
RaisePropertyChanged("IsShowPassword");
}
}
public bool IsRememberMe
{
get
{
return _isRememberMe;
}
set
{
_isRememberMe = value;
RaisePropertyChanged("IsRememberMe");
}
}
public bool IsShowPasswordIcon
{
get
{
return _isShowPasswordIcon;
}
set
{
_isShowPasswordIcon = value;
RaisePropertyChanged("IsShowPasswordIcon");
}
}
public ICommand TapCommandShowPassword
{
get { return _tapCommandShowPassword; }
}
public ICommand TapCommandNavigateForgotPassword
{
get { return _tapCommandNavigateForgotPassword; }
}
public ICommand Login
{
get { return _login; }
}
public void OnNavigatedFrom(NavigationParameters parameters)
{
}
public void OnNavigatedTo(NavigationParameters parameters)
{
}
public void OnNavigatingTo(NavigationParameters parameters)
{
}
void ShowHidePassword()
{
IsShowPassword = !IsShowPassword;
if (IsShowPassword)
IsShowPassword = true;
else
IsShowPassword = false;
}
void NavigateToForgotPassword()
{
_navigationService.NavigateAsync("ForgotPasswordPage");
}
async void LoginApiCalls(object s)
{
if (CrossConnectivity.Current.IsConnected)
{
try
{
UserDialogs.Instance.ShowLoading(ResX.AppResources.Loading_mesg, MaskType.Black);
if (_tapCommandNavigateForgotPassword != null)
{
DataManager = new DataManager(new RestService());
var resp = await DataManager.GetAuthToken(_password);
var isSuccess = false;
if (resp != null)
isSuccess = resp.IsSuccess;
if (isSuccess)
resp = await DataManager.GetUerAuth(_email, resp.Data);
if (resp.IsSuccess)
{
//store flag in shared pref
if (IsRememberMe)
{
Settings.Current.Set(Constants.PREF_IS_LOGGED_IN, true);
}
Settings.Current.Set(Constants.PREF_SALES_AGENT_ID, resp.SalesAgentId);
UserDialogs.Instance.HideLoading();
App.Current.MainPage = new NavigationPage(new HomePage());
//_navigationService.NavigateAsync("/HomePage");
}
else
{
await App.Current.MainPage.DisplayAlert(ResX.AppResources.Error_txt,
ResX.AppResources.UserName_Password_Error,ResX.AppResources.OK_txt);
UserDialogs.Instance.HideLoading();
}
}
}
catch (Exception e)
{
await App.Current.MainPage.DisplayAlert(ResX.AppResources.Error_txt,
ResX.AppResources.Generic_Error_txt, ResX.AppResources.OK_txt);
UserDialogs.Instance.HideLoading();
}
}
else
{
await App.Current.MainPage.DisplayAlert(ResX.AppResources.Net_Error_txt,
ResX.AppResources.No_Network_Error_txt, ResX.AppResources.OK_txt);
}
}
public void OnPhoneTapped(String phone)
{
var dialer = DependencyService.Get<IDialer>();
if (dialer != null)
{
dialer.Dial(phone);
}
}
public void OnRouteTap(string sourceLat, string sourceLon, string destinationLat, string destinationLon)
{
var route = DependencyService.Get<IDirection>();
if (route != null)
{
route.DrawDirection(sourceLat, sourceLon, destinationLat, destinationLon);
}
}
}
}
UNHANDLED EXCEPTION:
08-29 15:59:43.868 I/MonoDroid( 3268): System.NullReferenceException: Object reference not set to an instance of an object.
08-29 15:59:43.868 I/MonoDroid( 3268): at Xamarin.Forms.Platform.Android.AppCompat.NavigationPageRenderer.OnLayout (System.Boolean changed, System.Int32 l, System.Int32 t, System.Int32 r, System.Int32 b) [0x00007] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.Android\AppCompat\NavigationPageRenderer.cs:292
08-29 15:59:43.868 I/MonoDroid( 3268): at Xamarin.Forms.Platform.Android.FormsViewGroup.n_OnLayout_ZIIII (System.IntPtr jnienv, System.IntPtr native__this, System.Boolean p0, System.Int32 p1, System.Int32 p2, System.Int32 p3, System.Int32 p4) [0x00008] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Platform.Android.FormsViewGroup\obj\Release\generated\src\Xamarin.Forms.Platform.Android.FormsViewGroup.cs:198
08-29 15:59:43.868 I/MonoDroid( 3268): at (wrapper dynamic-method) System.Object:bbd5dc5a-77cf-45bd-9d16-717bbe7e4232 (intptr,intptr,bool,int,int,int,int)