undefined method `add_comment' for nil:NilClass (NoMethodError)

./tdiary/defaultio.rb:44:in `restore_comment'
./tdiary/defaultio.rb:35:in `each'
./tdiary/defaultio.rb:35:in `restore_comment'
./tdiary/defaultio.rb:33:in `open'
./tdiary/defaultio.rb:33:in `restore_comment'
./tdiary/defaultio.rb:144:in `transaction'
./tdiary.rb:1805:in `initialize'
index.rb:46:in `new'
index.rb:46