[Android] Fix WebView issue in EvaluateJavascriptAsync method using Shell

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue13326.cs

@@ -0,0 +1,95 @@
+using System;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 13326, "[Bug] [Android] WebView.EvaluateJavascriptAsync thrown NullReferenceException when is offscreen in Shell", PlatformAffected.Android)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.Shell)]
+#endif
+	public class Issue13326 : TestShell
+	{
+		const string Test1 = "Tab 1";
+		const string Test2 = "Tab 2";
+
+		protected override void Init()
+		{
+			AddBottomTab(CreatePage1(Test1), Test1);
+			AddBottomTab(CreatePage2(Test2), Test2);
+
+			static ContentPage CreatePage1(string title)
+			{
+				var layout = new StackLayout();
+
+				var instructions = new Label
+				{
+					Padding = 12,
+					BackgroundColor = Color.Black,
+					TextColor = Color.White,
+					Text = "Navigate to the second Tab"
+				};
+
+				layout.Children.Add(instructions);
+
+				return new ContentPage
+				{
+					Title = title,
+					Content = layout
+				};
+			}
+
+			static ContentPage CreatePage2(string title)
+			{
+				return new Issue13326SecondPage(title);
+			}
+		}
+	}
+
+	public class Issue13326SecondPage : ContentPage
+	{
+		WebView _webView;
+
+		public Issue13326SecondPage(string title)
+		{
+			Title = title;
+
+			var layout = new StackLayout();
+
+			var instructions = new Label
+			{
+				Padding = 12,
+				BackgroundColor = Color.Black,
+				TextColor = Color.White,
+				Text = "Navigate back to the first tab and wait some seconds, without crashing the test has passed."
+			};
+
+			_webView = new WebView
+			{
+				HeightRequest = 300,
+				WidthRequest = 300
+			};
+
+			layout.Children.Add(instructions);
+			layout.Children.Add(_webView);
+
+			Content = layout;
+
+			Task.Run(async () =>
+			{
+				await Task.Delay(TimeSpan.FromSeconds(5));
+
+				await _webView.EvaluateJavaScriptAsync("var a = 10;");
+			});
+		}
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -1802,6 +1802,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)FrameBackgroundIssue.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue14664.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue14192.xaml.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue13326.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue14697.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue8383.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue8383-2.xaml.cs" />

Xamarin.Forms.Core/WebView.cs

@@ -92,9 +92,12 @@ public void Eval(string script)
 
 		public async Task<string> EvaluateJavaScriptAsync(string script)
 		{
+			if (script == null)
+				return null;
+
 			EvaluateJavaScriptDelegate handler = EvaluateJavaScriptRequested;
 
-			if (script == null)
+			if (handler == null)
 				return null;
 
 			//make all the platforms mimic Android's implementation, which is by far the most complete.