add guard-code for users and visualizations

app/controllers/carto/api/vizjson3_presenter.rb

@@ -166,8 +166,9 @@ module Carto
       end
 
       def datasource_vizjson(options, forced_privacy_version)
+        username = @visualization.user.username unless @visualization.user.nil?
         ds = {
-          user_name: @visualization.user.username,
+          user_name: username,
           maps_api_template: ApplicationHelper.maps_api_template(api_templates_type(options)),
           stat_tag: @visualization.id
         }

app/services/carto/visualization_backup_service.rb

@@ -7,6 +7,8 @@ module Carto
     include ::LoggerHelper
 
     def create_visualization_backup(visualization:, category:, with_mapcaps: true, with_password: true)
+      return unless Carto::Visualization.exists?(id: visualization.id)
+
       export_json = export_visualization_json_hash(
         visualization.id,
         visualization.user,