In Themes/Flatly/layouts/master.blade.php in head, add a simple isset to alternate variable.
Old code:
@foreach($alternate as $alternateLocale=>$alternateSlug)
<link rel="alternate" hreflang="{{$alternateLocale}}" href="{{url($alternateLocale.'/'.$alternateSlug)}}">
@endforeach
New Code:
@if(isset($alternate))
@foreach($alternate as $alternateLocale=>$alternateSlug)
<link rel="alternate" hreflang="{{$alternateLocale}}" href="{{url($alternateLocale.'/'.$alternateSlug)}}">
@endforeach
@endif