NSView Tooltip property should allow setting null

[DavidKarlas]

Steps to Reproduce

1. new NSView().Tooltip = null;

Expected Behavior

Call native setTooltip: with nil

Actual Behavior

throw new ArgumentNullException ("value");

Environment

=== Visual Studio Community 2019 (Preview) for Mac ===

Version 8.2 Preview (8.2 build 386)
Installation UUID: 9bbbfbc6-8200-4909-a2e5-5d531e451a6f
	GTK+ 2.24.23 (Raleigh theme)
	Xamarin.Mac 5.6.0.25 (d16-0 / 50f75273)

	Package version: 518010026

=== Mono Framework MDK ===

Runtime:
	Mono 5.18.1.26 (pull-request-14392/d76505c27ee) (64-bit)
	Package version: 518010026

=== NuGet ===

Version: 5.0.2.5988

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	2.2.4
	2.1.10
	2.1.1
	2.1.0-preview1-25802-02
	2.0.0
	2.0.0-preview2-25407-01
SDK: /usr/local/share/dotnet/sdk/2.2.203/Sdks
SDK Versions:
	2.2.203
	2.1.603
	2.1.301
	2.1.0-preview1-007306
	2.1.0-preview1-006958
	2.0.0-preview3-006889
	2.0.0-preview2-006497
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.18.1/lib/mono/msbuild/Current/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.6.3
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Apple Developer Tools ===

Xcode 10.1 (14460.46)
Build 10B61

=== Xamarin.Mac ===

Version: 5.6.0.25 (Visual Studio Community)
Hash: 50f75273
Branch: d16-0
Build date: 2019-03-05 11:50:33-0800

=== Xamarin.Android ===

Version: 9.1.5.1 (Visual Studio Community)
Commit: HEAD/234003d1a
Android SDK: /Users/davidkarlas/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: 26.1.1
SDK Platform Tools Version: 26.0.0
SDK Build Tools Version: 27.0.0

=== Microsoft Mobile OpenJDK ===

Java SDK: /Users/davidkarlas/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 Device Manager ===

Version: 1.2.0.55
Hash: 642726c
Branch: remotes/origin/d16-2~1
Build date: 2019-05-21 15:24:32 UTC

=== Xamarin Inspector ===

Version: 1.4.3
Hash: db27525
Branch: 1.4-release
Build date: Mon, 09 Jul 2018 21:20:18 GMT
Client compatibility: 1

=== Xamarin.iOS ===

Version: 12.6.0.25 (Visual Studio Community)
Hash: 50f75273
Branch: d16-0
Build date: 2019-03-05 11:50:33-0800

=== Xamarin Designer ===

Version: 16.1.99.640
Hash: b48589cbe
Branch: remotes/origin/master
Build date: 2019-05-14 05:55:14 UTC

=== Build Information ===

Release ID: 802000386
Git revision: 34a93111b2bf726303e5906342b2680c11c09e37
Build date: 2019-05-22 13:31:13+00
Build branch: pr-david-dragDrop
Xamarin extensions: 19d1763237df85952f65b24fb18756fb05c0d614

=== Operating System ===

Mac OS X 10.14.4
Darwin 18.5.0 Darwin Kernel Version 18.5.0
    Mon Mar 11 20:40:32 PDT 2019
    root:xnu-4903.251.3~3/RELEASE_X86_64 x86_64

[Therzok]

This wasn't fixed?

[chamons]

I was mistaken, there are a number of tooltip PRs and I got them crosslinked.

[joconte]

I'm on it, I struggled with setting up the test environment but now It should be ok. I ping you guys on gitter when I'm done.

[Therzok]

There's a PR for it: #6119

[joconte]

@chamons @Therzok
Shouldn't we close this issue ?