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[.NET 6] [Source Compat Failure] ControlzEx app doesn't build with "Possible null reference" when retarget 6.0

See original GitHub issue

Application Name: ControlzEx (DiscordChatExporter failed with similar error code CS8601) OS: Windows 10 RS5 CPU: X64 .NET Build Number: 6.0.100-preview.1.21071.10

Verify Scenarios: 1). Windows 10 RS5 X64 + .NET Core SDK build 6.0.100-preview.1.21075.3: Fail 2). Windows 10 RS5 X64 + .NET Core SDK build 5.0.102: Pass

Repro steps Source code is targeting netcoreapp3.1 and net6.0-windows, global.json set to 6.0.100-preview.1.21075.3

Source Code at https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1269988
Make sure target 6.0
goto src\ControlzEx
dotnet build

Expected Result: Build succeeded

Actual Result:

C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\Theming\ThemeManager.cs(80,58): error CS8622: Nullability of reference types in type of parameter 'sender' of 'void ThemeManager.ThemesInternalCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)' doesn't match the target delegate 'NotifyCollectionChangedEventHandler' (possibly because of nullability attributes). [C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\ControlzEx_z4umz3jg_wpftmp.csproj]
C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\Theming\ThemeManager.cs(1235,59): error CS8622: Nullability of reference types in type of parameter 'sender' of 'void ThemeManager.HandleStaticPropertyChanged(object sender, PropertyChangedEventArgs e)' doesn't match the target delegate 'PropertyChangedEventHandler' (possibly because of nullability attributes). [C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\ControlzEx_z4umz3jg_wpftmp.csproj]
C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\Theming\ThemeManager.cs(1240,63): error CS8622: Nullability of reference types in type of parameter 'sender' of 'void ThemeManager.HandleStaticPropertyChanged(object sender, PropertyChangedEventArgs e)' doesn't match the target delegate 'PropertyChangedEventHandler' (possibly because of nullability attributes). [C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\ControlzEx_z4umz3jg_wpftmp.csproj]
C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\Theming\ThemeGenerator.cs(25,20): error CS8603: Possible null reference return. [C:\Users\valtunc\Desktop\AppSources\ControlzEx\src\ControlzEx\ControlzEx_z4umz3jg_wpftmp.csproj]

Results for DiscordChatExporter app:

C:\Users\valtunc\Desktop\AppSources\DiscordChatExporter\DiscordChatExporter.Gui\ViewModels\Framework\DialogScreen.cs(13,44): error CS8601: Possible null reference assignment. [C:\Users\valtunc\Desktop\AppSources\DiscordChatExporter\DiscordChatExporter.Gui\DiscordChatExporter.Gui_qfzufqja_wpftmp.csproj]

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