message: Fixed "Uncaught TypeError: Cannot read property 'id' of null" then single page becomes empty
diff: <!-- ko template: { name: 'survey-navigation'} -->
  <!-- /ko -->
  <!-- /ko -->
  + <!-- ko if: koCurrentPage -->
  <div data-bind="attr: { id: koCurrentPage().id }, template: { name: 'survey-page', data: koCurrentPage, afterRender: koAfterRenderPage }"></div>
  + <!-- /ko -->
  <div style="margin-top:1em" data-bind="visible: isShowProgressBarOnBottom, template: 'survey-progress'"></div>
  <!-- ko if: isTimerPanelShowingOnBottom -->
  <!-- ko template: { name: 'survey-timerpanel'} -->
  