Empty csv file while using BeanShell Postprocessor

i'm trying to save my variables into a csv file by using BeanShell Postprocessor, Code:

String id = "${userID}";
FileWriter fstream = new FileWriter("JmeterBean.csv",true);
fstream.write(id+"\n");
fstream.close();

Test Plan:

1. HTTP Request GetUsersById => return all IDs
2. Json extractor => name of variable userID
3. BeanShell Postprocessor

But my csv file is always empty and look like that: