ActiveRepository is sometimes set to null when changing PR branches

[jcansdale]

• GitHub Extension for Visual Studio version: v2.3.7.0
• Visual Studio version: Visual Studio 2015 (14.0.25420.1)

You can see what's going on from the log:

2018-01-17 12:13:00.060 INFO [38] TeamExplorerContext       Refresh ActiveRepository: RepositoryPath=C:\Source\github.com\grokys\PullRequestSandbox, BranchName=comments-test, HeadSha=5affff80a92fcb44367df4a1e4244882105ecafa
2018-01-17 12:13:00.060 INFO [38] TeamExplorerContext       Fire PropertyChanged event for ActiveRepository
2018-01-17 12:14:44.513 INFO [09] TeamExplorerContext       Refresh ActiveRepository: RepositoryPath=C:\Source\github.com\grokys\PullRequestSandbox, BranchName=pr/34-possible-missing-comment, HeadSha=a16aa3a7b1288d4c02b7859f1d2de83c10f65b2e
2018-01-17 12:14:44.513 INFO [09] TeamExplorerContext       Fire StatusChanged event for ActiveRepository
2018-01-17 12:14:49.277 INFO [154] TeamExplorerContext       Refresh ActiveRepository: RepositoryPath=null, BranchName=null, HeadSha=null
2018-01-17 12:14:49.277 INFO [154] TeamExplorerContext       Fire PropertyChanged event for ActiveRepository
2018-01-17 12:14:50.306 EROR [01] PullRequestDetailViewModel Error loading pull request grokys/PullRequestSandbox/34 from GitHub
System.NullReferenceException: Object reference not set to an instance of an object.
   at GitHub.Services.PullRequestService.<>c__DisplayClass22_0.<<EnsureLocalBranchesAreMarkedAsPullRequests>b__0>d.MoveNext() in C:\source\github.com\github\VisualStudio\src\GitHub.App\Services\PullRequestService.cs:line 267
--- End of stack trace from previous location where exception was thrown ---
   at System.Reactive.PlatformServices.ExceptionServicesImpl.Rethrow(Exception exception)
   at System.Reactive.ExceptionHelpers.ThrowIfNotNull(Exception exception)
   at System.Reactive.Subjects.AsyncSubject`1.GetResult()
   at GitHub.InlineReviews.Services.PullRequestSessionManager.<GetSession>d__14.MoveNext() in C:\source\github.com\github\VisualStudio\src\GitHub.InlineReviews\Services\PullRequestSessionManager.cs:line 148
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at GitHub.ViewModels.GitHubPane.PullRequestDetailViewModel.<Load>d__100.MoveNext() in C:\source\github.com\github\VisualStudio\src\GitHub.App\ViewModels\GitHubPane\PullRequestDetailViewModel.cs:line 344
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at GitHub.ViewModels.GitHubPane.PullRequestDetailViewModel.<Refresh>d__101.MoveNext() in C:\source\github.com\github\VisualStudio\src\GitHub.App\ViewModels\GitHubPane\PullRequestDetailViewModel.cs:line 448
2018-01-17 12:14:50.400 INFO [125] TeamExplorerContext       Refresh ActiveRepository: RepositoryPath=C:\Source\github.com\grokys\PullRequestSandbox, BranchName=pr/34-possible-missing-comment, HeadSha=a16aa3a7b1288d4c02b7859f1d2de83c10f65b2e
2018-01-17 12:14:50.400 INFO [125] TeamExplorerContext       Fire PropertyChanged event for ActiveRepository