code: @Override
    public Integer getScanCountAtMsLevel(int msLevel) {
      TreeMap<Integer, IScan> msLevelScanMap = getMapMsLevel2index().get(msLevel).getNum2scan();
      if (msLevelScanMap != null) {
        return msLevelScanMap.size();
      }
      return null;
    }
docs: If the msLevel doesn't exist in this ScanCollectionDefault, returns null
  
  @return null, if msLevel doesn't exist. Actual number of scans otherwise.