[isolated] Fix UnboundLocalError error
[isolated] Fix UnboundLocalError error

This is to fix https://chromium-swarm-dev.appspot.com/task?id=4dae5197818e9510

Bug: 1099655
Change-Id: I8decef1ce5ac34ae69ea318cf1644123356aab12
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2323210
Auto-Submit: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py