mapboxgl plugin with additional style URL's throws std::runtime_error

See attached zip for a minimal failing example.

An exception is thrown from Style::Impl::addLayer when switching between styles (use 1, 2 and 3 keys on your keyboard to do so) when the map server is unreachable (i.e. not running). Crashes every time.

Error output:

[ ERROR ] "{QSGRenderThread}[Setup]: loading style failed: Connection refused"
terminate called after throwing an instance of 'std::runtime_error'
what(): Layer QtLocation-12552752 already exists