Core: Guard against pages that forgot to create their widgets

This should not happen, but when it happens, the resulting crash is
quite cryptic. Make it a bit more obvious.

Change-Id: Icbe40206cfdbae93ccb072d7027fcaf109f2f39f
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/coreplugin/dialogs/ioptionspage.cpp

@@ -132,6 +132,7 @@ QWidget *IOptionsPage::widget()
     if (!m_widget) {
         if (m_widgetCreator) {
             m_widget = m_widgetCreator();
+            QTC_CHECK(m_widget);
         } else if (m_settingsProvider) {
             m_widget = new IOptionsPageWidget;
             AspectContainer *container = m_settingsProvider();

src/plugins/coreplugin/dialogs/settingsdialog.cpp

@@ -345,9 +345,12 @@ private:
     void showEvent(QShowEvent *event) final
     {
         if (!widget()) {
-            QWidget *inner = m_page->widget();
+            if (QWidget *inner = m_page->widget()) {
                 setWidget(inner);
                 inner->setAutoFillBackground(false);
+            } else {
+                QTC_CHECK(false);
+            }
         }
 
         QScrollArea::showEvent(event);