In the javascript configuration variables in mw.config, wgVisualEditorConfig is an object with among other properties a namespaces entry where the namespaces that VisualEditor can be used in are included. However, currently on meta this includes null, which isn't an actual namespace. It comes from the generating code in ApiVisualEditor::getAvailableNamespaceIds using null for namespaces that don't exist - these should be filtered out at the end.