Core: fix rare crash in GeneralSettingsWidget::apply()

The codec box might not contain any items
maybe the list failed to populate

found in https://the-qt-company-00.sentry.io/issues/6367744991

Pick-to: qds/4.7
Change-Id: I46acf97749d148edc45bd33d31815d7f7e96942b
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/coreplugin/generalsettings.cpp

@@ -295,6 +295,7 @@ void GeneralSettingsWidget::apply()
         setDpiPolicy(selectedPolicy);
     }
     currentIndex = m_codecBox->currentIndex();
+    if (currentIndex != -1)
         setCodecForLocale(m_codecBox->itemText(currentIndex).toLocal8Bit());
     // Apply the new base color if accepted
     StyleHelper::setBaseColor(m_colorButton->color());