PropertiesUserManager userDataFile is always NULL.

                PropertiesUserManagerFactory propUserManagerFactory = new PropertiesUserManagerFactory();
		propUserManagerFactory.setFile(new File("ftpusers.properties"));
		propUserManagerFactory.setPasswordEncryptor(new ClearTextPasswordEncryptor());
		mUserManager = (PropertiesUserManager) propUserManagerFactory.createUserManager();
		mFTPServerFactory.setUserManager(mUserManager);

PropertiesUserManager(passwordEncryptor, userDataFile,
                    adminName)
and you didn't set this.userDataFile = userDataFile;

  public PropertiesUserManager(PasswordEncryptor passwordEncryptor,
            File userDataFile, String adminName) {
        super(adminName, passwordEncryptor);
        
        this.userDataFile = userDataFile;
        
        loadFromFile(userDataFile);
    }