docs: @Override
      public void setActive(boolean active) {
          this.active = active;
  
          if (parent != null) {
              fireCollapsibleHandler();
              removeStyleName(CssName.ACTIVE);
              if (header != null) {
                  header.removeStyleName(CssName.ACTIVE);
              }
              if (active) {
                  if (parent != null && parent.isAccordion()) {
                      parent.clearActive();
                  }
                  addStyleName(CssName.ACTIVE);
  
                  if (header != null) {
                      header.addStyleName(CssName.ACTIVE);
                  }
              }
  
              if (body != null) {
                  body.setDisplay(active ? Display.BLOCK : Display.NONE);
              }
          } else {
              GWT.log("Please make sure that the Collapsible parent is attached or existed.", new IllegalStateException());
          }
      }