Primary constructor parameter is null after being run in a Task

public class PeriodicCallingIdentificationService(CallService callService)
{
    private Task? _executeTask;
    private CancellationTokenSource? _stoppingCts;
    private Modem? _modem;

    public void StartExecuting(CancellationToken cancellationToken)
    {
        if (_modem is null) throw new InvalidOperationException("Can't start executing without modem");
        _stoppingCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        _executeTask = Task.Run(async () =>
        {
            await DoWork(_stoppingCts.Token); // <-- here
            _modem.Close();
            _modem = null;
        }, cancellationToken);
    }