An Excel Importer test is failing in the finalizer

[alastairs]

From the build log:

[05:08:00][Step 2/4]
    Unhandled exception in remote appdomain: System.NullReferenceException: Object reference not set to an instance of an object.
       at GiveCRM.ImportExport.ExcelXlsImporter.Dispose(Boolean disposing) in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 117
       at GiveCRM.ImportExport.ExcelXlsImporter.Finalize() in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 104
[05:08:00][Step 2/4] 2012-04-08 05:08:00,488 [2328] ERROR JetBrains.TeamCity.NUnitLauncher.ErrorLoggerImpl - Unhandled exception in remote appdomain: System.NullReferenceException: Object reference not set to an instance of an object.
[05:08:00][Step 2/4]    at GiveCRM.ImportExport.ExcelXlsImporter.Dispose(Boolean disposing) in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 117
[05:08:00][Step 2/4]    at GiveCRM.ImportExport.ExcelXlsImporter.Finalize() in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 104
[05:08:00][Step 2/4] 2012-04-08 05:08:00,533 [2328] WARN  JetBrains.Container.Log4netDefaultLoggerInitializer - Unhandled exception in current domain: System.NullReferenceException: Object reference not set to an instance of an object.
[05:08:00][Step 2/4]    at GiveCRM.ImportExport.ExcelXlsImporter.Dispose(Boolean disposing) in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 117
[05:08:00][Step 2/4]    at GiveCRM.ImportExport.ExcelXlsImporter.Finalize() in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 104,    at GiveCRM.ImportExport.ExcelXlsImporter.Dispose(Boolean disposing) in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 117
[05:08:00][Step 2/4]    at GiveCRM.ImportExport.ExcelXlsImporter.Finalize() in c:\BuildAgent\work\d6628f96903cef80\src\GiveCRM.ImportExport\GiveCRM.ImportExport\ExcelXlsImporter.cs:line 104, (null)