Only load starter frontend decorators if they are available

Not everybody uses the starter frontend.

Author: tvdeyen

app/decorators/components/solidus_multi_domain/taxons_tree_component_decorator.rb

@@ -33,6 +33,8 @@ def tree(root_taxon:, item_classes:, current_item_classes:, max_level:)
       end
     end
 
-    ::TaxonsTreeComponent.prepend self
+    if const_defined?('::TaxonsTreeComponent')
+      ::TaxonsTreeComponent.prepend(self)
+    end
   end
 end

app/decorators/controllers/solidus_multi_domain/home_controller_decorator.rb

@@ -15,6 +15,8 @@ def index
       @new_arrivals = homepage_groups[3]
     end
 
-    ::HomeController.prepend(self)
+    if const_defined?('::HomeController')
+      ::HomeController.prepend(self)
+    end
   end
 end

app/decorators/controllers/solidus_multi_domain/products_controller_decorator.rb

@@ -18,6 +18,8 @@ def show
       @similar_products = @product.similar_products.select { |product| product.stores.include?(current_store) }
     end
 
-    ::ProductsController.prepend(self)
+    if const_defined?('::HomeController')
+      ::ProductsController.prepend(self)
+    end
   end
 end

app/decorators/controllers/solidus_multi_domain/taxons_controller_decorator.rb

@@ -10,6 +10,8 @@ def show
       @taxonomies = get_taxonomies
     end
 
-    ::TaxonsController.prepend(self)
+    if const_defined?('::TaxonsController')
+      ::TaxonsController.prepend(self)
+    end
   end
 end

app/decorators/controllers/solidus_multi_domain/users_controller_decorator.rb

@@ -7,6 +7,8 @@ def show
       @orders = @user.orders.complete.by_store(current_store).order('completed_at desc')
     end
 
-    ::UsersController.prepend(self)
+    if const_defined?('::UsersController')
+      ::UsersController.prepend(self)
+    end
   end
 end