收到此错误 - java.lang.NullPointerException：无法调用“com.aventstack.extentreports.ExtentReports.flush()”，因为“this.extent”为空

如何解决收到此错误 - java.lang.NullPointerException：无法调用“com.aventstack.extentreports.ExtentReports.flush()”，因为“this.extent”为空

package com.inetbanking.utilities;


import java.io.File;


import java.io.IOException;


import java.text.SimpleDateFormat;


import java.util.Date;


import org.testng.ITestContext;


import org.testng.ITestResult;


import org.testng.TestListenerAdapter;


import com.aventstack.extentreports.ExtentReports;


import com.aventstack.extentreports.ExtentTest;


import com.aventstack.extentreports.Status;


import com.aventstack.extentreports.markuputils.ExtentColor;


import com.aventstack.extentreports.markuputils.MarkupHelper;


import com.aventstack.extentreports.reporter.ExtentHtmlReporter;


import com.aventstack.extentreports.reporter.configuration.ChartLocation;


import com.aventstack.extentreports.reporter.configuration.Theme;


public class Reporting extends TestListenerAdapter





{
   public ExtentHtmlReporter htmlReporter;





   public ExtentReports extent;



   public ExtentTest logger;



    public void onstart(ITestContext testContext)



{
    String timeStamp = new SimpleDateFormat("yyyy.MM.dd.HH.mm.ss").format(new Date()); //Time Stamp





    String repName="Test-Report-"+timeStamp+".html";    
    htmlReporter=new ExtentHtmlReporter(System.getProperty("user.dir")+"/test-output/"+repName);  //specify location



    htmlReporter.loadXMLConfig(System.getProperty("user.dir")+"/extent-config.xml");    


     extent= new ExtentReports(); 


    extent.attachReporter(htmlReporter);


    extent.setSystemInfo("Host Name","localhost");


    extent.setSystemInfo("Environment","QA");


    extent.setSystemInfo("user","suresh");  


    htmlReporter.config().setDocumentTitle("cloudfy"); //title


    htmlReporter.config().setReportName("Functional Test Report");


    htmlReporter.config().setTestViewChartLocation(ChartLocation.TOP);


    htmlReporter.config().setTheme(Theme.DARK); }

     public void onTestSuccess(ITestResult tr)    {



        logger=extent.createTest(tr.getName());


        logger.log(Status.PASS,MarkupHelper.createLabel(tr.getName(),ExtentColor.GREEN));
        

        }  


  public void onTestFailure(ITestResult tr)  

  
    {
        logger=extent.createTest(tr.getName());


        logger.log(Status.FAIL,ExtentColor.RED));   




    
        String screenshotPath=System.getProperty("user.dir")+"\\Screenshots\\"+tr.getName()+".png";     




        File f=new File(screenshotPath);        
        if(f.exists())      {           


    try {
            


    logger.fail("screen shot is below:"+logger.addScreenCaptureFromPath(screenshotPath));
                }


            catch(IOException e)



            {



                e.printstacktrace();



            }


        }


    }



       public void onTestSkipped(ITestResult tr)


    {
        

logger=extent.createTest(tr.getName());



        logger.log(Status.SKIP,ExtentColor.ORANGE));



        
    } 



public void onFinish(ITestContext testContext) 

}


    {


        extent.flush(); 



    }




    
}