Error whith LogMessage in custom library

I develop a custom Library in C# to create a custom brick that interopt whith our SI.

In this custom code I want to use UiPath.Core.Activities.LogMessage to send error to the orchestrator but when i try this basic i have a nullreference :

LogMessage log = new LogMessage();
log.Message = “Log Message”;
log.Level = CurentLogLevel.Error;
WorkflowInvoker invoker = new WorkflowInvoker(log);

I think i have miss something but i do not know what and i have not found solution on this forum.