Fixing NPE when checking locked state.

- Regression from change 9f4ab32b1c62d16f03d14a50f13f04ddb58c541f.

Change-Id: I31ada9f4c051cf0aaa9a7bfa54c1b9bda1c64c16
Fixes: b/33738356
Test: Dock an app repeatedly
(cherry picked from commit b6b9950d13fd9f16be6ba7903d69f54a01639467)
1 file changed