output: #fixed code
                                  @Override
      public void startCrackedSession(ProtocolLibLoginSource source, StoredProfile profile, String username) {
          BukkitLoginSession loginSession = new BukkitLoginSession(username, profile);
          plugin.putSession(player.getAddress(), loginSession);
      }
input: #vulnerable code 
                                  @Override
      public void startCrackedSession(ProtocolLibLoginSource source, StoredProfile profile, String username) {
          BukkitLoginSession loginSession = new BukkitLoginSession(username, profile);
          plugin.getLoginSessions().put(player.getAddress().toString(), loginSession);
      } 
                              #location 4 
                              #vulnerability type NULL_DEREFERENCE