getting null exception while generating report using C# sparkhtmlreport specflow

Question

below code I am getting exception.first I am checking anyone help me on this

if (stepstatus == null)
{
    switch (steptype.ToString())
    {
        case "Given":
            scenario.CreateNode<Given>(stepName).Pass("passed");
            break;

here getting null exception scenario.CreateNode(stepName).Pass("passed");