Since #12241 we have notifications for this when the user navigates away from the viewer, but we do not yet warn when switching documents, which means that if you call e.g. `PDFViewerApplication.open` with a new document (or directly call `BaseViewer.setDocument` when using the viewer components standalone) there won't be any warning about losing form data. This should be addressed.
pdfViewer: null,
  
  pdfThumbnailViewer: null,
  
  pdfRenderingQueue: null,
  
  pdfPresentationMode: null,
  
  pdfDocumentProperties: null,
  
  pdfLinkService: null,
  
  pdfHistory: null,
  
  pdfSidebar: null,
  
  pdfSidebarResizer: null,
  
  pdfOutlineViewer: null,
  
  pdfAttachmentViewer: null,
  
  pdfLayerViewer: null,
  
  pdfCursorTools: null,
  
  pdfScriptingManager: null,
  
  store: null,