Redmine

I installed the plugin (QBurst Technologies) and edited lines in meeting_calendars_controller.rb as follows:

@project_id = "book-meeting-room-2" #This is the ID of my project (https://projets.pearsonerpi.com/projects/book-meeting-room-2)
@tracker_id = '8'   #This is the ID of Tracker Book Meeting (https://projets.pearsonerpi.com/trackers/8/edit)
@custom_field = CustomField.all    
@start_time = @custom_field[42].possible_values #This is the ID of Custom Field Start Time (https://projets.pearsonerpi.com/custom_fields/edit/42)
@end_time = @custom_field[43].possible_values  #This is the ID of Custom Field End Time (https://projets.pearsonerpi.com/custom_fields/edit/43)
@meeting_rooms=@custom_field[44].possible_values  #This is the ID of Custom Meeting Room (https://projets.pearsonerpi.com/custom_fields/edit/44)

When I go to https://projets.pearsonerpi.com/meeting_calendars I have an error 500:

NoMethodError (undefined method `possible_values' for nil:NilClass):
  vendor/plugins/redmine_meeting_room_calendar/app/controllers/meeting_calendars_controller.rb:9:in `initialize'

Can somebody tell me what's wrong as I really would like to use the plugin. Also I don't see what to change in lines 25 and 41 of the file:

25    if(week_day!=6 && week_day!=0)
41    recur_meeting -=1

Thanks.
Joseph