Missing null check in WindowsSnapshot.Dispose

[Jojo-1000]

• I have searched open and closed issues for duplicates.
• I have searched the forum for related topics.

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Environment info

• Duplicati version: 4.0.8.
• Operating system: Windows
• Backend: -

Description

WindowsSnapshot.Dispose() calls Dispose on a variable that is null if the constructor failed.

Steps to reproduce

1. Set snapshot policy to required
2. Run backup without admin permissions

• Actual result:
  Backup fails as expected, but verbose log shows cleanup error:

2024-05-14 18:29:20 +02 - [Verbose-Duplicati.Library.Snapshots.WindowsSnapshot-VSSCleanupOnError]: Failed during VSS error cleanup
System.NullReferenceException: Object reference not set to an instance of an object.
   at Duplicati.Library.Snapshots.WindowsSnapshot.Dispose(Boolean disposing) in Duplicati\Library\Snapshots\WindowsSnapshot.cs:line 324
   at Duplicati.Library.Snapshots.SnapshotBase.Dispose() in Duplicati\Library\Snapshots\SnapshotBase.cs:line 197
   at Duplicati.Library.Snapshots.WindowsSnapshot..ctor(IEnumerable`1 sources, IDictionary`2 options) in Duplicati\Library\Snapshots\WindowsSnapshot.cs:line 94

• Expected result:
  The cleanup should succeed, because nothing needs to be done.

[kenkendk]

Good find, it will be nice when #5166 is implemented