Error while trying to generate html report

For reference,

public class ExtentManager
{
public static ExtentHtmlReporter htmlReporter;

private static ExtentReports extent;
private ExtentManager()
{

}
public static ExtentReports getInstance()
{
if(extent == null)
{
String reportFile= DateTime.Now.ToString().Replace("/", "_").Replace(":", "_").Replace(" ", "_") + ".html";
htmlReporter = new ExtentHtmlReporter(@"D:\Selenium_files\Reports\" + reportFile);
extent = new ExtentReports();
extent.AttachReporter(htmlReporter);
extent.AddSystemInfo("OS", "Windows");
extent.AddSystemInfo("Host Name", "AiBATROS");
extent.AddSystemInfo("Environment", "QA");

string filePath = Path.GetDirectoryName(System.AppDomain.CurrentDomain.BaseDirectory);
filePath = Directory.GetParent(Directory.GetParent(filePath).FullName).FullName;
htmlReporter.LoadConfig(filePath + "\\extent-config.xml");

}
return extent;
}
}

Hello,

I solved the issue by downloading Extent Reports version 4

Earlier, which version were you using?