Fix potential crash when more than two alerts are requested.

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla43469.cs

@@ -16,7 +16,7 @@ namespace Xamarin.Forms.Controls
 	{
 		protected override void Init()
 		{
-			var button = new Button { Text = "Click to call DisplayAlert twice" };
+			var button = new Button { Text = "Click to call DisplayAlert three times" };
 
 			button.Clicked += (sender, args) =>
 			{
@@ -29,6 +29,11 @@ namespace Xamarin.Forms.Controls
 				{
 					await DisplayAlert("Second", "Text", "Cancel");
 				}));
+
+				Device.BeginInvokeOnMainThread(new Action(async () =>
+				{
+					await DisplayAlert("Three", "Text", "Cancel");
+				}));
 			};
 
 			Content = button;

Xamarin.Forms.Platform.UAP/PlatformUWP.cs

@@ -103,7 +103,7 @@ namespace Xamarin.Forms.Platform.UWP
 			if (options.Accept != null)
 				alertDialog.PrimaryButtonText = options.Accept;
 
-			if (s_currentAlert != null)
+			while (s_currentAlert != null)
 			{
 				await s_currentAlert;
 			}