[Android] Fix previewer exceptions with shell

[PureWeen]

Description of Change

This is a WIP but its purpose was to get through all the Forms SDK exceptions to the point where the previewer would just run shell or not

Current exception coming from previewer

[2019-04-18 16:32:31.5] ERROR: : The renderer encountered an error while rendering this file: System.NullReferenceException: Object reference not set to an instance of an object
Xamarin.Forms.Previewer.XamarinFormsLoaderAndroid.GetViewGroup (System.Object renderer)
AndroidFormsRendering.XFPageRendererView.SetupView (MonoTouch.Design.XamlRenderContext xamlContext)
AndroidFormsRendering.XFPageRendererView.Initialize (System.String xamlId)

[2019-04-18 16:32:31.5] ERROR: : AndroidXamlRenderer had a rendering error: System.NullReferenceException: Object reference not set to an instance of an object
Xamarin.Forms.Previewer.XamarinFormsLoaderAndroid.GetViewGroup (System.Object renderer)
AndroidFormsRendering.XFPageRendererView.SetupView (MonoTouch.Design.XamlRenderContext xamlContext)
AndroidFormsRendering.XFPageRendererView.Initialize (System.String xamlId)

Issues Resolved

• fixes Xamarin.Forms Previewer : NullReference Exception with Shell #5908

API Changes

None

Platforms Affected

• Core/XAML (all platforms)
• iOS
• Android
• UWP

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

PR Checklist

• Has automated tests
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

VS bug #854002

[PureWeen]

@garuma the other area I'm not sure about is how to use a FragmentManager when running as designer

the context type provided from designer is

com.android.layoutlib.bridge.android.BridgeContext

Shell/TabbedPage/MDP all retrieve the SupportFragmentManager from the ACtivity

Xamarin.Forms/Xamarin.Forms.Platform.Android/Renderers/ShellRenderer.cs

Line 154 in 441cb3a

private FragmentManager FragmentManager => ((FormsAppCompatActivity)AndroidContext.GetActivity()).SupportFragmentManager;

and that's how they load the content

but I don't have a fragment activity when executing under the context of designer. Is there another approach we can take that's designer friendly?

[PureWeen]

Alright this PR gets rid of all the exceptions with shell and previewer but it's still displaying a blank page most likely because of these errors

[2019-04-29 16:03:53.9] Renderer >> Caused by: java.lang.IllegalArgumentException: The style on this component requires your app theme to be Theme.AppCompat (or a descendant).

[2019-04-29 16:03:52.6] Renderer >> ERROR: An exception occured while loading control android.support.design.widget.AppBarLayout
[2019-04-29 16:03:52.7] Renderer >> ERROR: java.lang.reflect.InvocationTargetException

I'm thinking we need to either theme wrap our controls when using previewer or previewer needs to use an appcompat theme

for now we can at least review and merge this PR into 4.0.0 to fix the exceptions

[PureWeen]

Once I activated the appcompat theme for this previewer started throwing a whole new set of exceptions

so I set this to DNM for now

[PureWeen]

alright @samhouts @rmarinho this one is ready to go (Android only)

you should see a glorious blank page when loading a shell page into previewer.

Once we get appcompat enabled that should also load as blank as well

[drewgillies]

This lgtm--one line to look at though.