code: protected void open() {
  
          m_oldValue = m_selectionInput.m_textbox.getValue();
          if (m_popup == null) {
              String title = org.opencms.gwt.client.Messages.get().key(
                  org.opencms.gwt.client.Messages.GUI_GALLERY_SELECT_DIALOG_TITLE_0);
              m_popup = new CmsFramePopup(title, buildGalleryUrl());
  
              m_popup.setCloseHandler(new Runnable() {
  
                  public void run() {
  
                      String textboxValue = m_selectionInput.m_textbox.getText();
  
                      if (!m_oldValue.equals(textboxValue)) {
                          m_selectionInput.m_textbox.setValue("", true);
                          m_selectionInput.m_textbox.setValue(textboxValue, true);
                      }
  
                      if (m_previewHandlerRegistration != null) {
                          m_previewHandlerRegistration.removeHandler();
                          m_previewHandlerRegistration = null;
                      }
                      m_selectionInput.m_textbox.setFocus(true);
                      m_selectionInput.m_textbox.setCursorPos(m_selectionInput.m_textbox.getText().length());
                  }
              });
              m_popup.setModal(false);
              m_popup.setId(m_id);
              m_popup.setWidth(717);
              m_popup.getFrame().setSize("705px", "640px");
              m_popup.addDialogClose(new Command() {
  
                  public void execute() {
  
                      close();
  
                  }
              });
          } else {
              m_popup.getFrame().setUrl(buildGalleryUrl());
          }
          m_popup.setAutoHideEnabled(true);
          m_popup.center();
          if (m_previewHandlerRegistration == null) {
              m_previewHandlerRegistration = Event.addNativePreviewHandler(new CloseEventPreviewHandler());
          }
      }
docs: Opens the popup of this widget.<p>