error happend when view daemon errors pages

hello, I'm running fluentd-ui v1.1.0, fluentd 1.2.6, when accessing daemon errors page,

browser shows

We're sorry, but something went wrong.
If you are the application owner check the logs for more information.

and fluentd ui log shows following information:

I, [2018-10-13T18:40:43.458550 #9920] INFO -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] Started GET "/daemon/errors" for 127.0.0.1 at 2018-10-13 18:40:43 +0800
I, [2018-10-13T18:40:43.464120 #9920] INFO -- : [86c2bd27-7048-47ea-8f66-1277919082dd] Completed 200 OK in 56ms (Views: 0.6ms)
I, [2018-10-13T18:40:43.467729 #9920] INFO -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] Processing by FluentdController#errors as HTML
I, [2018-10-13T18:40:43.536376 #9920] INFO -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] Completed 500 Internal Server Error in 68ms
F, [2018-10-13T18:40:43.537201 #9920] FATAL -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e]
F, [2018-10-13T18:40:43.537254 #9920] FATAL -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] NoMethodError (undefined method `[]' for nil:NilClass):
F, [2018-10-13T18:40:43.537283 #9920] FATAL -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e]
F, [2018-10-13T18:40:43.537321 #9920] FATAL -- : [a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:115:in `block in split_error_lines_to_error_units'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:113:in `each'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:113:in `each_with_index'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:113:in `split_error_lines_to_error_units'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:75:in `block in logged_errors'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:74:in `block in split_each_line'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:73:in `each'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:73:in `split_each_line'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:48:in `read_rest'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:15:in `block in each_line'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:13:in `loop'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] lib/file_reverse_reader.rb:13:in `each_line'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:58:in `logged_errors'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/models/fluentd_log.rb:16:in `errors_since'
[a8c60824-9bc3-4a19-bd35-8d42f6f9e20e] app/controllers/fluentd_controller.rb:44:in `errors'

seems it happend after I've sent some utf8 log to fluentd (I'm not sure about that), and this problem can't recovery by restart fluentd-ui . any suggestion ?