Peter,
Not sure if you were aware, but an attempted install of either PDFViewer v01.00.06 addon (net1, net2) throws the same error on DNN 4.3.5 as follows.

EndJob Files created 
StartJob Registering DesktopModule
Failure

ExceptionSystem.Data.SqlClient.SqlException: Cannot insert the value NULL into column 'DesktopModuleID', table 'DED977ESWA.dbo.DesktopModules'; column does not allow nulls. INSERT fails. The statement has been terminated. at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection) at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) at System.Data.SqlClient.SqlDataReader.ConsumeMetaData() at System.Data.SqlClient.SqlDataReader.get_MetaData() at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString) at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async) at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result) at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) at System.Data.SqlClient.SqlCommand.ExecuteScalar() at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteScalar(SqlConnection connection, CommandType commandType, String commandText, SqlParameter[] commandParameters) at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteScalar(String connectionString, CommandType commandType, String commandText, SqlParameter[] commandParameters) at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteScalar(String connectionString, String spName, Object[] parameterValues) at DotNetNuke.Data.SqlDataProvider.AddDesktopModule(String ModuleName, String FolderName, String FriendlyName, String Description, String Version, Boolean IsPremium, Boolean IsAdmin, String BusinessControllerClass, Int32 SupportedFeatures) at DotNetNuke.Entities.Modules.DesktopModuleController.AddDesktopModule(DesktopModuleInfo objDesktopModule) at DotNetNuke.Modules.Admin.ResourceInstaller.PaDnnInstallerBase.RegisterModules(PaFolder Folder, ArrayList Modules, ArrayList Controls) at DotNetNuke.Modules.Admin.ResourceInstaller.PaDnnInstallerBase.Install(PaFolderCollection folders) at DotNetNuke.Modules.Admin.ResourceInstaller.PaInstaller.Install()