An error “Package System.ArgumentNullException: Value cannot be null” occurs when restoring package with the context menu: ‘Restore NuGet Packages’ after cleaning solution

[CiciLi1]

NuGet Product Used

Visual Studio Package Management UI

Product Version

NuGet Client Dev\6.11.0.55

Worked before?

It’s a regression since it doesn’t repro on Main\34915.142 + NuGet Client Dev\6.11.0.50.

Impact

It's more difficult to complete my work

Repro Steps & Context

Repro Steps:   

1. Create a C# Console App (.NET Core) project in VS.

2. Right-click the solution node in Solution Explorer window and click the context menu: ‘Clean Solution’.

3. Right-click the solution node in Solution Explorer window and click the context menu: ‘Restore NuGet Packages’.

Expected:   

The retore is successful without any error.

Actual:

An error “Package System.ArgumentNullException: Value cannot be null” occurs as below:

Notes:    

1. The repro rate is 100%.    

2. It doesn’t repro on non-SDK project that install package with “packages.config” format.

3. It doesn’t repro after running ‘dotnet restore’ or ‘msbuild /t:restore’ command in CLI and ‘nuget restore’ in PMC.

Verbose Logs

No response

[nkolev92]

Team Triage: If we don't fix this by EOD, we should revert the change.

@jebriede, please hold the insertion until this is fixed or reverted.

[nkolev92]

Same fix applies to both #13451 and this.

I have the change ready, it was a simple fix. I'm just gonna add some more tests before PRing.