code: public Document getDocument(@NonNull String id) {
          if (id == null) { throw new IllegalArgumentException("id cannot be null."); }
  
          synchronized (lock) {
              mustBeOpen();
              try {
                  return new Document((Database) this, id, false);
              }
              catch (CouchbaseLiteException ex) {
                  // only 404 - Not Found error throws CouchbaseLiteException
                  return null;
              }
          }
      }
docs: Gets an existing Document object with the given ID. If the document with the given ID doesn't
  exist in the database, the value returned will be null.
  
  @param id the document ID
  @return the Document object