code: @Nonnull
    public MindMapPanel lock() {
      if (this.panelLocker != null) {
        this.panelLocker.lock();
        if (this.isDisposed()) {
          this.panelLocker.unlock();
          throw new IllegalStateException("Mind map has been already disposed!");
        }
      }
      return this;
    }
docs: Lock the panel.
  
  @return the panel
  @throws IllegalStateException it will be thrown if the panel is disposed