NullReferenceException is thrown in PackageImportTreeView when importing a .unitypackage containing ./ prefixed paths on Windows

How to reproduce:

1. Create a new project
2. Navigate to Assets -> Import Package -> Custom Package...
3. Select the attached "GooglePlayGamesPlugin-2.3.0.unitypackage"
4. Observe the Console and the Import Unity Package window

Actual result: NullReferenceException is thrown and the Import Unity Package window is empty
Expected result: The Import Unity Package window lists the package contents and the package can be imported

Reproducible with: 6000.0.0f1, 6000.0.85f1, 6000.3.26f1, 6000.6.0a6
Not reproducible with: 6000.6.6f1, 6000.7.0b5, 7000.0.0a8
Fixed in: 6000.6.0a7

Reproducible on: Windows 10 (by user), Windows 11
Not reproducible on: MacOS 26.7.1 (M3 Max)

Workaround: Manually extract and repack the .unitypackage without the ./ prefix in the internal archive paths

Notes:

• On 6000.0.0f1 no error is thrown, but the Import Unity Package window does not open and nothing is imported
• The package's archive entries start with a "./" prefix (e.g. "./1c5d1cb1974d14c0c8b32fcf00089556/") and include a root "./" entry
• Error: “NullReferenceException: Object reference not set to an instance of an object UnityEditor.PackageImportTreeView.RecursiveComputeEnabledStateForFolders”