Crash when Close Viewmodel With Result using MasterDetail

[juanagu]

Sample repository: https://github.com/juanagu/MvvmCross/tree/master

I Added:
ViewModel : MixedNavResultDetailViewModel
Page: MixedNavResultDetailPage

Steps to reproduce 📜

1. Run Playground.Forms.iOS or Playground.Forms.Android

2. Open Show Mixed Navigation

3. Click Login

4. In Menu Drawer selected "Result" option.

5. Click "Cerrar"

Expected behavior 🤔

Close viewmodel and return Result

Actual behavior 🐛

Application Crash:

System.NullReferenceException: Object reference not set to an instance of an object
  at MvvmCross.Forms.Views.MvxFormsPagePresenter.CloseMasterDetailPage (MvvmCross.Core.ViewModels.IMvxViewModel viewModel, MvvmCross.Forms.Views.Attributes.MvxMasterDetailPagePresentationAttribute attribute) [0x0004b] in /Users/jagu/Projects/GitHub/MvvmCross/MvvmCross-Forms/MvvmCross.Forms/Views/MvxFormsPagePresenter.cs:438 
  at MvvmCross.Forms.Views.MvxFormsPagePresenter.<RegisterAttributeTypes>b__19_5 (MvvmCross.Core.ViewModels.IMvxViewModel viewModel, MvvmCross.Core.Views.MvxBasePresentationAttribute attribute) [0x00000] in /Users/jagu/Projects/GitHub/MvvmCross/MvvmCross-Forms/MvvmCross.Forms/Views/MvxFormsPagePresenter.cs:126 
  at MvvmCross.Core.Views.MvxAttributeViewPresenter.Close (MvvmCross.Core.ViewModels.IMvxViewModel viewModel) [0x00057] in /Users/jagu/Projects/GitHub/MvvmCross/MvvmCross/Core/Core/Views/MvxAttributeViewPresenter.cs:183 
  at MvvmCross.Core.Views.MvxAttributeViewPresenter.ChangePresentation (MvvmCross.Core.ViewModels.MvxPresentationHint hint) [0x0001e] in /Users/jagu/Projects/GitHub/MvvmCross/MvvmCross/Core/Core/Views/MvxAttributeViewPresenter.cs:162 
  at MvvmCross.Forms.Views.MvxFormsPagePresenter.<>n__0 (MvvmCross.Core.ViewModels.MvxPresentationHint hint) <0x130e27020 + 0x0005a> in <5bdbd8d89a4c4dd5bd6f8f025b1007a7>:0 
  at MvvmCross.Forms.Views.MvxFormsPagePresenter+<ChangePresentation>d__20.MoveNext () [0x00435] in /Users/jagu/Projects/GitHub/MvvmCross/MvvmCross-Forms/MvvmCross.Forms/Views/MvxFormsPagePresenter.cs:205

I debug the code and the line "var masterDetailHost = GetPageOfType();" on MvxFormsPagePresenter return null

Configuration 🔧

Version: 5.7.0

Platform:

• 🐒 Xamarin.Forms

[nickrandolph]

@juanagu a couple of requests (mainly for future issues)

• Can you submit your example as a PR instead of just a remote repo
• Can you please make sure your example is based off current develop (ie v6 at the moment)