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
fCurrentSelection= null;
		if (hasChanges()) {
			fPageContainer.showPage(fStandardPage);
			ChangeElement treeViewerInput= (ChangeElement)fTreeViewer.getInput();
			if (visible && treeViewerInput != null) {
				IStructuredSelection selection= (IStructuredSelection)fTreeViewer.getSelection();
				if (selection.isEmpty()) {
					ITreeContentProvider provider= (ITreeContentProvider)fTreeViewer.getContentProvider();
					ChangeElement element= getFirstNonCompositeChange(provider, treeViewerInput);
					if (element != null) {
						if (getRefactoringWizard().internalGetExpandFirstNode(InternalAPI.INSTANCE)) {
							Object[] subElements= provider.getElements(element);
							if (subElements != null && subElements.length > 0) {
								fTreeViewer.expandToLevel(element, 999);
							}
						}
						fTreeViewer.setSelection(new StructuredSelection(element));
					}
				}
			}
			fFilterDropDownAction.initialize(collectGroupCategories());
			super.setVisible(visible);
			fTreeViewer.getControl().setFocus();
		} else {
			fPageContainer.showPage(fNullPage);
			super.setVisible(visible);
		}
		getRefactoringWizard().internalSetPreviewShown(InternalAPI.INSTANCE, visible);
	}