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