FEATURE: make artisan command for installing work with submodules

app/Models/Module.php

@@ -2,10 +2,8 @@
 
 namespace App\Models;
 
-use App\Handlers\Contracts\AppHandler;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Support\Facades\App;
-use Modules\Blog\Article\Handlers\Article;
 
 /**
  * Class Module
@@ -44,7 +42,7 @@ class Module extends Model
             throw new \ErrorException('Modules can be installed only via Artisan!');
         }
 
-        $managesContent = in_array('App\Handlers\Contracts\AppHandler', class_implements($handler));
+        $managesContent = $handler ? in_array('App\Handlers\Contracts\AppHandler', class_implements($handler)) : false;
         $supportCategories = $managesContent ? call_user_func([$handler, 'supportCategories']) : false;
 
         // insert module into database
@@ -60,7 +58,9 @@ class Module extends Model
                                  ]);
 
         // install something module-specific
-        call_user_func([$handler, 'install']);
+        if ($handler) {
+            call_user_func([$handler, 'install']);
+        }
 
         return $module;
     }