Package Manager throws ScriptableSingleton related errors after a package was installed while there were pending package manifest changes

Steps to reproduce:

1. Open the attached project
2. Open the Package Manager window
3. Select one of the local packages
4. Expand the Manage menu and select the Edit Manifest option
5. Make any change to the package's manifest
6. Without saving or discarding changes, switch to Packman's Unity Registry tab
7. Install any package or feature (For example - the 2D feature)
8. Observe the Console window

Actual results: SerializedObject  and ScriptableSingleton already exists errors are thrown

Expected results: No errors or warnings are thrown after installing a package or a feature

Reproducible with versions: 6000.0.73f1, 6000.3.13f1, 6000.4.3f1, 6000.5.0b5, 6000.6.0a3

Tested on (OS): macOS 26.3.1

Notes:

• ScriptableSingleton-related errors will persist and will be thrown after installing/removing subsequent packages. It will disappear after reloading the Editor, however.