The big NULL replacement party part 3.

Getting there I think.

src/ScreenManager.cpp

@@ -332,14 +332,14 @@ void ScreenManager::ReloadOverlayScreens()
 Screen *ScreenManager::GetTopScreen()
 {
 	if( g_ScreenStack.empty() )
-		return NULL;
+		return nullptr;
 	return g_ScreenStack[g_ScreenStack.size()-1].m_pScreen;
 }
 
 Screen *ScreenManager::GetScreen( int iPosition )
 {
 	if( iPosition >= (int) g_ScreenStack.size() )
-		return NULL;
+		return nullptr;
 	return g_ScreenStack[iPosition].m_pScreen;
 }