Then we need to implement the method that will open and close the Panel...

CustomFormPanelCommandSet_PART2.ts

// Other code here...

import * as React from 'react';
import * as ReactDom from 'react-dom';

// Other code here...


export default class CustomFormPanelCommandSet extends BaseListViewCommandSet<ICustomFormPanelCommandSetProperties> {

  // Other code here...
  
  private _showPanel(itemId: number, currentTitle: string) {
    this._renderPanelComponent({
      isOpen: true,
      currentTitle,
      itemId,
      listId: this.context.pageContext.list.id.toString(),
      onClose: this._dismissPanel
    });
  }

  @autobind
  private _dismissPanel() {
    this._renderPanelComponent({ isOpen: false });
  }

  private _renderPanelComponent(props: any) {
    const element: React.ReactElement<ICustomPanelProps> = React.createElement(CustomPanel, assign({
      onClose: null,
      currentTitle: null,
      itemId: null,
      isOpen: false,
      listId: null
    }, props));
    ReactDom.render(element, this.panelPlaceHolder);
  }
}