According to you, you are able to accept the alert but test step is showing exception in console :
You can catch the exception and proceed further with your next steps:
Below is the code : 
        try {
            Alert myAlert = driver.switchTo().alert();
            myAlert.accept();
        } catch (Exception e) {
            System.out.println("######### Successfully Accepted Alert ################");
        }
// here proceed with your next steps.  
Hope it will help you out.
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solved When I run test in selenium using java, the alert gets closed but in code it shows failure