Bug 93400 LTK UI should provide official API for structural content in the preview page
org.eclipse.ltk.ui.refactoring/src/org/eclipse/ltk/internal/ui/refactoring/PreviewWizardPage.java
if (element == null) {
				showNullPreviewer();
			} else {
				ChangePreviewViewerDescriptor descriptor= element.getChangePreviewViewerDescriptor();
				if (fCurrentDescriptor != descriptor) {
					IChangePreviewViewer newViewer;
					if (descriptor != null) {
						newViewer= descriptor.createViewer();
						newViewer.createControl(fPreviewContainer);
					} else {
						newViewer= fNullPreviewer;
					}
					fCurrentDescriptor= descriptor;
					element.feedInput(newViewer, fActiveGroupCategories);
					if (fCurrentPreviewViewer != null && fCurrentPreviewViewer != fNullPreviewer)
						fCurrentPreviewViewer.getControl().dispose();
					fCurrentPreviewViewer= newViewer;				
					fPreviewContainer.showPage(fCurrentPreviewViewer.getControl());
				} else {
					element.feedInput(fCurrentPreviewViewer, fActiveGroupCategories);
				}
			}
		} catch (CoreException e) {
			showNullPreviewer();
			ExceptionHandler.handle(e, getShell(),
						RefactoringUIMessages.PreviewWizardPage_refactoring, 
						RefactoringUIMessages.PreviewWizardPage_Internal_error); 
		}
	}