chore: API fixes (#3014)

- Minor API fixes
- Configuration screen for LINE inbox

app/controllers/api/v1/accounts/inboxes_controller.rb

@@ -100,6 +100,7 @@ class Api::V1::Accounts::InboxesController < Api::V1::Accounts::BaseController
   end
 
   def update_channel_feature_flags
+    return unless @inbox.web_widget?
     return unless permitted_params(Channel::WebWidget::EDITABLE_ATTRS)[:channel].key? :selected_feature_flags
 
     @inbox.channel.selected_feature_flags = permitted_params(Channel::WebWidget::EDITABLE_ATTRS)[:channel][:selected_feature_flags]