Intermittent 'undefined method log_exception for nil' error.

[zjmletang]

commit 9faca5e (HEAD, tag: v0.17.4)

The log link is provided below. The issue disappeared after a rerun, and the reproduction pattern remains unknown (it might be related to concurrency testing)

58120_pvpanic_2016.txt

[kostyanf14]

Should be fixed in master by 2a7398a