Random error when import file in read_cache

[polargeex]

Completed 500 Internal Server Error in 429ms (ActiveRecord: 57.3ms)

NameError (uninitialized class variable @@cache in MsprojImpController
Did you mean? catch):
plugins/msproject_import/app/controllers/msproj_imp_controller.rb:308:in read_cache' lib/redmine/sudo_mode.rb:63:in sudo_mode'

I solve it by added one line at start of read_cahe but it's dirty

def read_cache @@cache = ActiveSupport::Cache::FileStore.new(Rails.root.join('tmp','msproj_imp').to_s) @resources = @@cache.read(:resources)

Projet.zip
Redmin 3.2.3 with mysql
Debian 8
Rails 4.2.5.2
ruby 2.3.4p301 (2017-03-30 revision 58214) [x86_64-linux]