Fixed migration erroring (#1681)

db/migrate/20200413150518_add_role_id_to_users.rb

@@ -16,7 +16,7 @@ class AddRoleIdToUsers < ActiveRecord::Migration[5.2]
 
         MigrationProduct.where(role_id: nil).each do |user|
           highest_role = SubMigrationProduct.joins("INNER JOIN users_roles ON users_roles.role_id = roles.id")
-                                            .where("users_roles.user_id = '#{user.id}'").min_by(&:priority).id
+                                            .where("users_roles.user_id = '#{user.id}'")&.min_by(&:priority)&.id
           user.update_attributes(role_id: highest_role) unless highest_role.nil?
         end
       end