Enable nullable reference checking via .props file.

The Nullable annotations on netstandard2.0 are incomplete and incorrect in places. Ignore nullable warnings on netstandard2.0 and make them errors on later target frameworks.

build/NullableEnable.props

@@ -0,0 +1,11 @@
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <!-- 
+    The Nullable annotations on netstandard2.0 are incomplete and incorrect in places. Ignore
+    nullable warnings on netstandard2.0 and make them errors on later target frameworks.
+  -->
+  <PropertyGroup>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors Condition="'$(TargetFramework)' != 'netstandard2.0'">$(WarningsAsErrors);nullable</WarningsAsErrors>
+    <NoWarn Condition="'$(TargetFramework)' == 'netstandard2.0'">$(NoWarn);nullable</NoWarn>
+  </PropertyGroup>
+</Project>

src/Avalonia.Diagnostics/Avalonia.Diagnostics.csproj

@@ -3,7 +3,6 @@
     <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <RootNamespace>Avalonia</RootNamespace>
     <PackageId>Avalonia.Diagnostics</PackageId>
-    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Update="**\*.xaml.cs">
@@ -34,4 +33,5 @@
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\BuildTargets.targets" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>

src/Avalonia.Input/Avalonia.Input.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
-    <Nullable>Enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\Avalonia.Base\Metadata\NullableAttributes.cs" Link="NullableAttributes.cs" />
@@ -15,4 +14,5 @@
   </ItemGroup>  
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>

src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj

@@ -3,8 +3,6 @@
     <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <PackageId>Avalonia.ReactiveUI</PackageId>
     <SignAssembly>false</SignAssembly>
-    <Nullable>enable</Nullable>
-    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\packages\Avalonia\Avalonia.csproj" />
@@ -12,4 +10,5 @@
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ReactiveUI.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>

src/Markup/Avalonia.Markup/Avalonia.Markup.csproj

@@ -2,8 +2,6 @@
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <RootNamespace>Avalonia</RootNamespace>
-    <Nullable>Enable</Nullable>
-    <WarningsAsErrors>CS8600;CS8602;CS8603</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <None Remove="Markup\Parsers\Nodes\ExpressionGrammer" />
@@ -19,4 +17,5 @@
   <Import Project="..\..\..\build\Rx.props" />
   <Import Project="..\..\..\build\System.Memory.props" />
   <Import Project="..\..\..\build\ApiDiff.props" />
+  <Import Project="..\..\..\build\NullableEnable.props" />
 </Project>

src/Web/Avalonia.Web.Blazor/Avalonia.Web.Blazor.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
-    <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <PackageId>Avalonia.Web.Blazor</PackageId>
     <LangVersion>preview</LangVersion>
@@ -32,7 +31,8 @@
   <Import Project="..\..\..\build\BuildTargets.targets" />
   <Import Project="..\..\..\build\SkiaSharp.props" />
   <Import Project="..\..\..\build\HarfBuzzSharp.props" />
-
+  <Import Project="..\..\..\build\NullableEnable.props" />
+  
   <ItemGroup>
     <Content Include="*.props">
       <Pack>true</Pack>