docs: @Override
      public void cleanUpNullReferences() {
          for (Map.Entry<K, V> entry : map.entrySet()) {
              Object entryVal = entry.getValue();
              if (entryVal instanceof SoftReference && ((SoftReference) entryVal).get() == null) {
                  map.remove(entry.getKey(), entryVal);
              }
          }
      }