Settings: don't crash when dismissing FRP

If the user has pressed home while the dailog is up, there may be
nothing to dismiss by the time the task finishes.

Ref: CYNGNOS-427
Change-Id: Ic4fdec1ecb32f88181feba4bfd3eb337b1b61efd
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit 54f4854c)