see https://github.com/TracksApp/tracks/pull/191

error:

NoMethodError (undefined method `+' for nil:NilClass):
app/controllers/stats_controller.rb:380:in `block (2 levels) in convert_to_array'
app/controllers/stats_controller.rb:380:in `each'
app/controllers/stats_controller.rb:380:in `block in convert_to_array'
app/controllers/stats_controller.rb:380:in `each'
app/controllers/stats_controller.rb:380:in `convert_to_array'
app/controllers/stats_controller.rb:385:in `put_events_into_month_buckets'
app/controllers/stats_controller.rb:20:in `actions_done_last12months_data'