NSView.NextKeyView is not allowed to be null

[iainx]

Steps to Reproduce

Set NextKeyView to null

view.NextKeyView = null;

Expected Behavior

NextKeyView is allowed to be null. From https://developer.apple.com/documentation/appkit/nsview/1483465-nextkeyview?language=objc

The value in this property is nil if there is no next view in the key view loop.

Actual Behavior

System.ArgumentNullException: Value cannot be null.
Parameter name: value
at AppKit.NSView.set_NextKeyView (AppKit.NSView value)

Environment

=== Visual Studio Enterprise 2017 for Mac (Preview) ===

Version 7.7 Preview (7.7 build 557)
Installation UUID: f64c47d3-1a2d-4092-8fae-dc3309136470
Runtime:
Mono 5.14.0.105 (2018-04/6d28c8f9621) (64-bit)
GTK+ 2.24.23 (Raleigh theme)
Xamarin.Mac 4.4.1.178 (master / eeaeb7e)

Package version: 514000105

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
2.0.0
1.1.1
1.1.0
1.0.4
SDK: /usr/local/share/dotnet/sdk/2.0.0/Sdks
SDK Versions:
2.0.0
1.0.1
1.0.0-preview2-1-003177
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.14.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.0.0
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Apple Developer Tools ===

Xcode 9.4.1 (14161)
Build 9F2000

=== Xamarin.Mac ===

Version: 4.4.1.178 (Visual Studio Enterprise)
Hash: eeaeb7e
Branch: master
Build date: 2018-05-14 17:36:06-0400

=== Xamarin.iOS ===

Version: 11.5.0.92 (Visual Studio Enterprise)
Hash: d0bb1c5
Branch: master
Build date: 2017-09-27 11:43:16-0400

=== Xamarin.Android ===

Version: 8.3.0.19 (Visual Studio Enterprise)
Android SDK: /Users/iain/Library/Developer/Xamarin/android-sdk-macosx
Supported Android versions:
4.1 (API level 16)
4.2 (API level 17)
4.3 (API level 18)
4.4 (API level 19)
4.4.87 (API level 20)
5.0 (API level 21)
5.1 (API level 22)
6.0 (API level 23)
7.0 (API level 24)
7.1 (API level 25)
8.0 (API level 26)
8.1 (API level 27)

SDK Tools Version: 25.2.5
SDK Platform Tools Version: 28
SDK Build Tools Version: 27.0.0

Java SDK: /usr
java version "1.8.0_101"
Java(TM) SE Runtime Environment (build 1.8.0_101-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)

Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android Device Manager ===

Version: 1.0.629.0
Hash: ccfe436f

=== Xamarin Inspector ===

Version: 1.4.2
Hash: 213565e
Branch: 1.4-release
Build date: Thu, 05 Jul 2018 17:45:17 GMT
Client compatibility: 1

=== Build Information ===

Release ID: 707000557
Git revision: b838dcbbb3a55cc2ef268020b2c99e661daa251e
Build date: 2018-07-09 12:18:08+00
Build branch: master
Xamarin extensions: f96a1e6f9b7806a813caeaec415d7ba33de3dcba

=== Operating System ===

Mac OS X 10.13.5
Darwin 17.6.0 Darwin Kernel Version 17.6.0
Tue May 8 15:22:16 PDT 2018
root:xnu-4570.61.1~1/RELEASE_X86_64 x86_64