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

./tdiary/defaultio.rb:40:in `restore_comment'
./tdiary/defaultio.rb:30:in `open'
./tdiary/defaultio.rb:30:in `restore_comment'
./tdiary/defaultio.rb:134:in `transaction'
./tdiary.rb:1577:in `initialize'
index.rb:44:in `new'
index.rb:44