why NullPointerException appears in java programm while second time initing bufferreader? How can I fix it?

Question

I created program in java. which reads files from several ftp servers. But it reads first file by bufferreader and second time occures NullPointerException. Here is my code:

    while(true){//READ .CSV AND WRITE TO DB
        client=new FTPClient();
        try {
            client.connect(ftps.getIp());
            client.enterLocalActiveMode();
            client.login(ftps.getLogin(),ftps.getPassword());
            System.out.println("Connected with ftp "+ftps.getName());
            String[] files = client.listNames();
            InputStream ins;
            BufferedReader bf;
            for(int i=0;i<files.length;i++){
                ins=client.retrieveFileStream(files[i]);
                bf= new BufferedReader(new InputStreamReader(ins));
                try {
                    String str = "";
                    do
                    {
                        lines.add(str);                            
                        str = bf.readLine();
                    } while (!str.equals(null));
                }catch (NullPointerException ex){                        
                }
                catch (Exception ex){
                    System.out.println("Error while reading file "+files[i]+" from ftp "+ftps.getName()+" ERROR "+ex.toString());
                }
                finally {
                    bf.close();                        
                }
            }
        } catch (Exception e) {
            System.out.println("Error in executing ftp "+ftps.getName()+e.toString());

        }finally {
            if(client.isConnected())
                try{
                    client.disconnect();
                }catch (Exception ex){
                    System.out.println("Error while disconnecting with  ftp "+ftps.getName());
                }
                System.out.println("Disconnected with ftp "+ftps.getName());
        }

Error in executing ftp ftpserver's name appeares in console.