Repro steps:
- start a new project with 2021.2.0a20
- open package Manager -> Unity Registry
- select the Mobile Feature Set
- notice an empty line - there should be the Adaptive Performance package there, but the line is just empty

When selecting the empty line, you'll get this error in the console:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PackageManager.UI.Internal.FeatureDependencies.OnDependencyItemClicked (UnityEditor.PackageManager.UI.Internal.IPackageVersion version) (at <77a45f0c92274da6a584cead7046b27a>:0)
UnityEditor.PackageManager.UI.Internal.FeatureDependencies+<>c__DisplayClass12_0.<SetPackageVersion>b__0 () (at <77a45f0c92274da6a584cead7046b27a>:0)
UnityEditor.PackageManager.UI.Internal.VisualElementExtensions+<>c__DisplayClass0_0.<OnLeftClick>b__0 (UnityEngine.UIElements.MouseDownEvent e) (at <77a45f0c92274da6a584cead7046b27a>:0)