adsk-forge-LevelSectionPanel-step3&4

async createUI() {
    this.uiCreated = true;
    const table = document.createElement( 'table' );
    table.className = 'adsk-lmv-tftable adn-lvl-section-panel-table';

    const tbody = document.createElement( 'tbody' );
    table.appendChild( tbody );
    this.scrollContainer.appendChild( table );

    const upperRow = tbody.insertRow( -1 );
    const upperTextCell = upperRow.insertCell( 0 );
    upperTextCell.innerText = 'Upper:';
    const upperSelectCell = upperRow.insertCell( 1 );

    const lowerRow = tbody.insertRow( -1 );
    const lowerTextCell = lowerRow.insertCell( 0 );
    lowerTextCell.innerText = 'Lower:';
    const lowerSelectCell = lowerRow.insertCell( 1 );

    // ...

    const upperLvlSelector = document.createElement( 'select' );
    upperLvlSelector.id = 'adn-upper-lvl-selector';
    upperLvlSelector.className ='adn-lvl-selector';
    upperSelectCell.appendChild( upperLvlSelector );

    const lowerLvlSelector = document.createElement( 'select' );
    lowerLvlSelector.id = 'adn-lower-lvl-selector';
    lowerLvlSelector.className = 'adn-lvl-selector';
    lowerSelectCell.appendChild( lowerLvlSelector );

    const data = await this.getRemoteLevels();
    this.levels = data;

    this.createSelectOptions( data, upperLvlSelector );
    this.createSelectOptions( data, lowerLvlSelector );

    // ...

    const buttonRow = tbody.insertRow( -1 );
    const buttonCell = buttonRow.insertCell( 0 );
    buttonCell.colSpan = 2;

    const sectionButton = document.createElement( 'button' );
    sectionButton.type = 'button';
    sectionButton.textContent = 'Apply';
    buttonCell.appendChild( sectionButton );
    sectionButton.addEventListener( 'click', this.onButtonClicked );

    this.resizeToContent();
}