[UWP] MasterDetailControl will no longer null out the TCS before it is used. (#227)

* [UWP] Don't null the _commandBarTcs too soon

* Add repro for 41842

3 files changed, 43 insertions, 7 deletions

diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla41842.cs b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla41842.cs
new file mode 100644
index 00000000..f5714233
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla41842.cs
@@ -0,0 +1,35 @@
+using System;
+
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 41842, "Set MasterDetailPage.Detail = New Page() twice will crash the application when set MasterBehavior = MasterBehavior.Split", PlatformAffected.WinRT)]
+	public class Bugzilla41842 : TestMasterDetailPage
+	{
+		protected override void Init()
+		{
+			MasterBehavior = MasterBehavior.Split;
+
+			Master = new Page() { Title = "Master" };
+
+			Detail = new NavigationPage(new Page());
+			Detail = new NavigationPage(new ContentPage { Content = new Label { Text = "Success" } });
+		}
+
+#if UITEST
+		[Test]
+		public void Bugzilla41842Test()
+		{
+			RunningApp.WaitForElement(q => q.Marked("Success"));
+		}
+#endif
+	}
+}
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
index 5c963249..1e9d1ea0 100644
--- a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
@@ -153,6 +153,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla39458.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla39853.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)TestPages\ScreenshotConditionalApp.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla41842.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)_Template.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1028.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1075.cs" />
diff --git a/Xamarin.Forms.Platform.UAP/MasterDetailControl.cs b/Xamarin.Forms.Platform.UAP/MasterDetailControl.cs
index 9ec03f8d..6787ccec 100644
--- a/Xamarin.Forms.Platform.UAP/MasterDetailControl.cs
+++ b/Xamarin.Forms.Platform.UAP/MasterDetailControl.cs
@@ -167,7 +167,11 @@ namespace Xamarin.Forms.Platform.UWP
 
 			_commandBarTcs = new TaskCompletionSource<CommandBar>();
 			ApplyTemplate();
-			return _commandBarTcs.Task;
+
+			var commandBarFromTemplate = _commandBarTcs.Task;
+			_commandBarTcs = null;
+
+			return commandBarFromTemplate;
 		}
 
 		protected override void OnApplyTemplate()
@@ -193,12 +197,8 @@ namespace Xamarin.Forms.Platform.UWP
 
 			UpdateMode();
 
-			TaskCompletionSource<CommandBar> tcs = _commandBarTcs;
-			if (tcs != null)
-			{
-				_commandBarTcs = null;
-				tcs.SetResult(_commandBar);
-			}
+			if (_commandBarTcs != null)
+				_commandBarTcs.SetResult(_commandBar);
 		}
 
 		static void OnShouldShowSplitModeChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)