How to reproduce:
1. Create a new Unity project
2. Go to Window -> Package Management -> Package Manager -> Unity Registry
3. Search for the Multiplayer Play Mode and click "Install"
4. Observe the results
Expected results: No errors are thrown and the package is installed
Actual results: ArgumentNullException is thrown and the package is marked as installed while it is not actually installed
Reproducible with: 3.0.0 (6000.6.0a7)
Not reproducible with: 2.0.2 (6000.5.0b11)
Reproducible on: macOS Tahoe 26 (M1)
Not reproducible on: No other environment tested
Notes:
- In early 6.6 alphas the package is not installed but no error is thrown
- The package is successfully installed via Multiplayer Center setup
First lines of error:
ArgumentNullException: Value cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[TKey,TValue].FindEntry (TKey key) (at <b3de1de343a349299daf7e70e6f0b89b>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].ContainsKey (TKey key) (at <b3de1de343a349299daf7e70e6f0b89b>:0)
UnityEditor.SceneView.CleanAdditionalSettingsOnPackageRemoval (UnityEditor.PackageManager.PackageRegistrationEventArgs args) (at /Users/bokken/build/output/unity/unity/Editor/Mono/SceneView/SceneView.cs:1147)