Core: Use Id() instead of 0 for invalid modes.

Fixes a regression introduced in 5e47d35.

Change-Id: I73dbf34008b616ee70b3ac8024af0197b8273a83
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/modemanager.cpp

@@ -119,7 +119,7 @@ Id ModeManager::currentMode()
 {
     int currentIndex = d->m_modeStack->currentIndex();
     if (currentIndex < 0)
-        return 0;
+        return Id();
     return d->m_modes.at(currentIndex)->id();
 }