Core: Fix compilation

Change-Id: Icae9a1db31e887c751b3a0c71cfd93b4fa6ead98
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/coreplugin/themesettingswidget.cpp

@@ -156,7 +156,7 @@ void ThemeSettingsWidget::refreshThemeList()
 
 void ThemeSettingsWidget::apply()
 {
-    const int index = d->m_ui->themeComboBox->currentIndex();
+    const int index = d->m_themeComboBox->currentIndex();
     if (index == -1)
         return;
     const QString themeId = d->m_themeListModel->themeAt(index).id().toString();