macOS share extension doesn't show any interface

### Steps to Reproduce

1. Build a project with share extension (sample is provided but it works with any project)
2. Try to share from Safari
3.

### Expected Behavior
The user interface of the share extension is shown.

### Actual Behavior
Nothing happens.

### Environment

```
=== Visual Studio Community 2019 for Mac ===

Version 8.9.1 (build 34)
Installation UUID: f558466e-e717-453b-99e2-e900e934b179
GTK+ 2.24.23 (Raleigh theme)
Xamarin.Mac 6.18.0.23 (d16-6 / 088c73638)

Package version: 612000122

=== Mono Framework MDK ===

Runtime:
Mono 6.12.0.122 (2020-02/c621c35ffa0) (64-bit)
Package version: 612000122

=== Xamarin Designer ===

Version: 16.9.0.316
Hash: bd2705417
Branch: remotes/origin/d16-9
Build date: 2021-02-24 00:16:08 UTC

=== Roslyn (Language Service) ===

3.9.0-6.21152.10+c10f884b30737542ddd84ca889a4aad9281ce210

=== NuGet ===

Version: 5.8.0.6860

=== .NET Core SDK ===

SDK: /usr/local/share/dotnet/sdk/5.0.201/Sdks
SDK Versions:
5.0.201
5.0.103
5.0.102
5.0.101
5.0.100
3.1.407
3.1.406
3.1.405
3.1.404
3.1.403
3.1.402
3.1.401
3.1.302
3.1.301
3.1.300
3.1.202
3.1.200
3.1.102
3.1.101
3.1.100
3.0.101
3.0.100
2.1.701
2.1.700
MSBuild SDKs: /Applications/Visual Studio.app/Contents/Resources/lib/monodevelop/bin/MSBuild/Current/bin/Sdks

=== .NET Core Runtime ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
5.0.4
5.0.3
5.0.2
5.0.1
5.0.0
3.1.13
3.1.12
3.1.11
3.1.10
3.1.9
3.1.8
3.1.7
3.1.6
3.1.5
3.1.4
3.1.2
3.1.1
3.1.0
3.0.1
3.0.0
2.1.23
2.1.22
2.1.21
2.1.20
2.1.19
2.1.18
2.1.17
2.1.16
2.1.15
2.1.14
2.1.13
2.1.12
2.1.11

=== .NET Core 3.1 SDK ===

SDK: 3.1.407

=== Xamarin.Profiler ===

Version: 1.6.12.26
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Apple Developer Tools ===

Xcode 12.4 (17801)
Build 12D4e

=== Xamarin.Mac ===

Version: 7.8.2.5 (Visual Studio Community)
Hash: 3836759d4
Branch: d16-9
Build date: 2021-02-10 17:56:43-0500

=== Xamarin.iOS ===

Version: 14.14.2.5 (Visual Studio Community)
Hash: 3836759d4
Branch: d16-9
Build date: 2021-02-10 17:56:44-0500

=== Xamarin.Android ===

Version: 11.2.0.21 (Visual Studio Community)
Commit: xamarin-android/d16-9/93eab59
Android SDK: /Users/ivanicin/Library/Developer/Xamarin/android-sdk-macosx
Supported Android versions:
None installed

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 28.0.2
SDK Build Tools Version: 28.0.3

Build Information:
Mono: 5e9cb6d
Java.Interop: xamarin/java.interop/d16-9@d6d86b2
ProGuard: Guardsquare/proguard/v7.0.1@912d149
SQLite: xamarin/sqlite/3.34.1@daff8f4
Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-9@9d8924d

=== Microsoft OpenJDK for Mobile ===

Java SDK: /Users/ivanicin/Library/Developer/Xamarin/jdk/microsoft_dist_openjdk_1.8.0.25
1.8.0-25
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android SDK Manager ===

Version: 16.9.0.22
Hash: a391de2
Branch: remotes/origin/d16-9
Build date: 2021-02-18 03:14:56 UTC

=== Android Device Manager ===

Version: 16.9.0.17
Hash: fc2b3db
Branch: remotes/origin/d16-9
Build date: 2021-02-18 03:15:18 UTC

=== Build Information ===

Release ID: 809010034
Git revision: 33bce2b7f3d45d05865cd3282f1c2cdc3a112aa0
Build date: 2021-03-04 11:21:38-05
Build branch: release-8.9
Xamarin extensions: 33bce2b7f3d45d05865cd3282f1c2cdc3a112aa0

=== Operating System ===

Mac OS X 10.16.0
Darwin 20.3.0 Darwin Kernel Version 20.3.0
Thu Jan 21 00:07:06 PST 2021
root:xnu-7195.81.3~1/RELEASE_X86_64 x86_64

```

### Build Logs

### Example Project (If Possible)

[sharetest.zip](https://github.com/xamarin/xamarin-macios/files/6164523/sharetest.zip)