docs: public void cleanUpNullReferences() {
          synchronized (map) {
              final Iterator<Map.Entry<K, V>> iterator = map.entrySet().iterator();
              while (iterator.hasNext()) {
                  final Map.Entry<K, V> entry = iterator.next();
                  final Object value = entry.getValue();
  
                  if (!(value instanceof SoftReference)) continue;
                  if (((SoftReference) value).get() == null) iterator.remove();
              }
          }
      }