/usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/tmux/client.rb:12:in `version': undefined method `[]' for nil:NilClass (NoMethodError)
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/tmux.rb:69:in `_check_available'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/base.rb:59:in `initialize'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:100:in `new'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:100:in `_add'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:62:in `block (2 levels) in detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:60:in `each'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:60:in `detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:60:in `block in detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:59:in `each'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:59:in `detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:180:in `_detect_or_add_notifiers'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:198:in `_activate'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:87:in `initialize'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:42:in `new'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:42:in `connect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/notifier.rb:11:in `connect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard.rb:61:in `setup'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/commander.rb:32:in `start'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/cli/environments/valid.rb:16:in `start_guard'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/cli.rb:122:in `start'
        from /usr/local/rvm/gems/ruby-2.4.1@global/gems/thor-0.20.0/lib/thor/command.rb:27:in `run'
        from /usr/local/rvm/gems/ruby-2.4.1@global/gems/thor-0.20.0/lib/thor/invocation.rb:126:in `invoke_command'
        from /usr/local/rvm/gems/ruby-2.4.1@global/gems/thor-0.20.0/lib/thor.rb:387:in `dispatch'
        from /usr/local/rvm/gems/ruby-2.4.1@global/gems/thor-0.20.0/lib/thor/base.rb:466:in `start'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/aruba_adapter.rb:32:in `execute'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/aruba_adapter.rb:19:in `execute!'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/bin/_guard-core:11:in `<main>'