Options: Fix restoring last page for "delayed" created categories

E.g. when opening Designer > Forms, restarting Qt Creator and opening
options dialog again.

Change-Id: I0cfabed8c9e7ef8c98abab5490181223558e21a2
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/coreplugin/dialogs/settingsdialog.cpp

@@ -365,6 +365,7 @@ void SettingsDialog::showPage(Id categoryId, Id pageId)
         if (category->id == initialCategory) {
             initialCategoryIndex = i;
             if (initialPage.isValid()) {
+                ensureCategoryWidget(category);
                 for (int j = 0; j < category->pages.size(); ++j) {
                     IOptionsPage *page = category->pages.at(j);
                     if (page->id() == initialPage)