[Bug] Shell throws NullRef if improperly configured

[Eilon]

Description

While I don't expect Shell to work if improperly configured, I don't expect a NullRef in these cases either (NullRef is nearly always invalid if there are no bugs).

Steps to Reproduce

Minimal case where Shell works (good):

            var shell = new Shell();
            var shellContent = new ShellContent();
            shell.Items.Add(shellContent);

            MainPage = shell;

Verbose case where Shell works (good):

            var shell = new Shell();
            var tabBar = new TabBar();
            var tab = new Tab();
            var shellContent = new ShellContent();
            shellContent.Content = new ContentPage() { Content = new Label() { Text = "HI!!!" } };
            tab.Items.Add(shellContent);
            tabBar.Items.Add(tab);
            shell.Items.Add(tabBar);

            MainPage = shell;

But if you "partially" configure Shell, you get NullRef exceptions. For example, add a TabBar with a Tab but no ShellContent (bad):

            var shell = new Shell();
            var tabBar = new TabBar();
            var tab = new Tab();
            tabBar.Items.Add(tab);
            shell.Items.Add(tabBar);

            MainPage = shell;

And you get this:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

 	0xFFFFFFFFFFFFFFFF in System.Diagnostics.Debugger.Mono_UnhandledException_internal	C#
 	0x1 in System.Diagnostics.Debugger.Mono_UnhandledException at /Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/mcs/class/corlib/System.Diagnostics/Debugger.cs:120,4	C#
 	0x33 in Android.Runtime.DynamicMethodNameCounter.45	C#
 	0xB3 in Xamarin.Forms.Platform.Android.ShellSectionRenderer.OnCreateView at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellSectionRenderer.cs:150,4	C#
 	0x24 in Android.Support.V4.App.Fragment.n_OnCreateView_Landroid_view_LayoutInflater_Landroid_view_ViewGroup_Landroid_os_Bundle_	C#
 	0x23 in Android.Runtime.DynamicMethodNameCounter.45	C#
 	0xFFFFFFFFFFFFFFFF in Java.Interop.NativeMethods.java_interop_jnienv_call_nonvirtual_void_method_a	C#
 	0x79 in Java.Interop.JniEnvironment.InstanceMethods.CallNonvirtualVoidMethod	C#
 	0x5F in Java.Interop.JniPeerMembers.JniInstanceMethods.InvokeVirtualVoidMethod	C#
 	0x12 in Android.App.Activity.OnStart	C#
 	0x20 in Xamarin.Forms.Platform.Android.FormsAppCompatActivity.OnStart at D:\a\1\s\Xamarin.Forms.Platform.Android\AppCompat\FormsAppCompatActivity.cs:360,4	C#
 	0xA in Android.App.Activity.n_OnStart	C#
 	0x11 in Android.Runtime.DynamicMethodNameCounter.21	C#

Another "partially" configure Shell that causes a NullRef exception: Add a TabBar without a Tab or ShellContent (bad):

            var shell = new Shell();
            var tabBar = new TabBar();
            shell.Items.Add(tabBar);

            MainPage = shell;

And you get this:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

 	0xFFFFFFFFFFFFFFFF in System.Diagnostics.Debugger.Mono_UnhandledException_internal	C#
 	0x1 in System.Diagnostics.Debugger.Mono_UnhandledException at /Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/mcs/class/corlib/System.Diagnostics/Debugger.cs:120,4	C#
 	0x33 in Android.Runtime.DynamicMethodNameCounter.45	C#
 	0xD3 in Xamarin.Forms.Platform.Android.ShellBottomNavViewAppearanceTracker.SetBackgroundColor at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellBottomNavViewAppearanceTracker.cs:90,5	C#
 	0x27 in Xamarin.Forms.Platform.Android.ShellBottomNavViewAppearanceTracker.ResetAppearance at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellBottomNavViewAppearanceTracker.cs:33,4	C#
 	0xC in Xamarin.Forms.Platform.Android.ShellItemRenderer.ResetAppearance at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellItemRenderer.cs:335,47	C#
 	0x12 in Xamarin.Forms.Platform.Android.ShellItemRenderer.Xamarin.Forms.IAppearanceObserver.OnAppearanceChanged at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellItemRenderer.cs:41,5	C#
 	0x1A in Xamarin.Forms.Shell.Xamarin.Forms.IShellController.AddAppearanceObserver at D:\a\1\s\Xamarin.Forms.Core\Shell\Shell.cs:203,4	C#
 	0xBC in Xamarin.Forms.Platform.Android.ShellItemRenderer.OnCreateView at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellItemRenderer.cs:76,4	C#
 	0x24 in Android.Support.V4.App.Fragment.n_OnCreateView_Landroid_view_LayoutInflater_Landroid_view_ViewGroup_Landroid_os_Bundle_	C#
 	0x23 in Android.Runtime.DynamicMethodNameCounter.45	C#
 	0xFFFFFFFFFFFFFFFF in Java.Interop.NativeMethods.java_interop_jnienv_call_nonvirtual_void_method_a	C#
 	0x79 in Java.Interop.JniEnvironment.InstanceMethods.CallNonvirtualVoidMethod	C#
 	0x5F in Java.Interop.JniPeerMembers.JniInstanceMethods.InvokeVirtualVoidMethod	C#
 	0x12 in Android.App.Activity.OnStart	C#
 	0x20 in Xamarin.Forms.Platform.Android.FormsAppCompatActivity.OnStart at D:\a\1\s\Xamarin.Forms.Platform.Android\AppCompat\FormsAppCompatActivity.cs:360,4	C#
 	0xA in Android.App.Activity.n_OnStart	C#
 	0x11 in Android.Runtime.DynamicMethodNameCounter.21	C#

But if you just create an empty Shell with nothing at all, you get a much nicer exception (good):

            var shell = new Shell();

            MainPage = shell;

You get this nice exception:

System.ArgumentException: 'Active Shell Item not set. Have you added any Shell Items to your Shell?
Parameter name: ShellItem'

 	0xFFFFFFFFFFFFFFFF in System.Diagnostics.Debugger.Mono_UnhandledException_internal	C#
 	0x1 in System.Diagnostics.Debugger.Mono_UnhandledException at /Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/mcs/class/corlib/System.Diagnostics/Debugger.cs:120,4	C#
 	0x33 in Android.Runtime.DynamicMethodNameCounter.45	C#
 	0x80 in Xamarin.Forms.Platform.Android.ShellItemRenderer.OnCreateView at D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ShellItemRenderer.cs:70,5	C#
 	0x24 in Android.Support.V4.App.Fragment.n_OnCreateView_Landroid_view_LayoutInflater_Landroid_view_ViewGroup_Landroid_os_Bundle_	C#
 	0x23 in Android.Runtime.DynamicMethodNameCounter.45	C#
 	0xFFFFFFFFFFFFFFFF in Java.Interop.NativeMethods.java_interop_jnienv_call_nonvirtual_void_method_a	C#
 	0x79 in Java.Interop.JniEnvironment.InstanceMethods.CallNonvirtualVoidMethod	C#
 	0x5F in Java.Interop.JniPeerMembers.JniInstanceMethods.InvokeVirtualVoidMethod	C#
 	0x12 in Android.App.Activity.OnStart	C#
 	0x20 in Xamarin.Forms.Platform.Android.FormsAppCompatActivity.OnStart at D:\a\1\s\Xamarin.Forms.Platform.Android\AppCompat\FormsAppCompatActivity.cs:360,4	C#
 	0xA in Android.App.Activity.n_OnStart	C#
 	0x11 in Android.Runtime.DynamicMethodNameCounter.21	C#

Expected Behavior

More consistent and nice exceptions for various incorrectly-configured Shell instances.

Actual Behavior

NullRef thrown from incorrectly-configured Shell instances.

Basic Information

• Version with issue: Xamarin.Forms 4.3.0