NullReferenceException in SSE stream of process engine controller

[1nf0rmagician]

Describe the bug
The process engine controller runs into NullReferenceExceptions while streaming ProcessHolderGroup events

An unhandled exception has occurred while executing the request.
System.NullReferenceException: Object reference not set to an instance of an object.
at Moryx.ControlSystem.Processes.Endpoints.StreamServices.ProcessHolderGroupStream.Start(HttpContext context, CancellationToken cancelToken) in /home/runner/work/moryx-controlsystem/moryx-controlsystem/src/Moryx.ControlSystem.Processes.Endpoints/StreamServices/ProcessHolderGroupStream.cs:line 55
at Moryx.ControlSystem.Processes.Endpoints.ProcessEngineController.GroupStream(CancellationToken cancelToken) in /home/runner/work/moryx-controlsystem/moryx-controlsystem/src/Moryx.ControlSystem.Processes.Endpoints/ProcessEngineController.cs:line 335
at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskResultExecutor.Execute(ActionContext actionContext, IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location ---