invoiceninja/app/Providers/AppServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Database\Eloquent\Relations\Relation;
use Illuminate\Support\Facades\Blade;
use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        Relation::morphMap([
            'invoices' => '\App\Models\Invoice',
            'proposals' => '\App\Models\Proposal',
        ]);

        Blade::if('env', function($environment){
            return config('ninja.environment') === $environment;
        });
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }
}