Steps to reproduce:

1. Create a new Universal 3D Project from Hub
2. Hold Ctrl and move the Package Manager button elsewhere in the Toolbar
3. Click it to open the Package Manager
4. Close the Package Manager
5. Repeat steps 2-3 two to four times (Opening PM while Packages are loading, then at some point the Packages load while the Window is opened)

Actual results: 2 Identical Errors are logged: "[Package Manager Window] Package manager extension failed with error: System.NullReferenceException: Object reference not set to an instance of an object
  at SampleDependencyImporter.get_panelRoot () [0x00001] in .\Library\PackageCache\com.unity.render-pipelines.core@2e3e104bd146\Editor\SampleDependencyImportSystem\SampleDependencyImporter.cs:51 
  at SampleDependencyImporter.RefreshSampleButtons () [0x00036] in .\Library\PackageCache\com.unity.render-pipelines.core@2e3e104bd146\Editor\SampleDependencyImportSystem\SampleDependencyImporter.cs:91 
  at SampleDependencyImporter.UnityEditor.PackageManager.UI.IPackageManagerExtension.OnPackageSelectionChange (UnityEditor.PackageManager.PackageInfo packageInfo) [0x0005a] in .\Library\PackageCache\com.unity.render-pipelines.core@2e3e104bd146\Editor\SampleDependencyImportSystem\SampleDependencyImporter.cs:171"

Expected results: No errors are logged

Reproducible with versions: 6000.3.0a6, 6000.4.0b1, 6000.4.0b5, 6000.5.0a5

Not reproducible with versions: 6000.0.66f1, 6000.3.0a5

Tested on (OS): Windows 11