WiFi: Fix Null Pointer Exception in Wifi Settings dialog

This patch checks that the activity remains available in the
dialog onclick callback before using it.

Change-Id: I1886b87498455e890962415a864da803c94561a4
Orig-Change-Id: I1d7aba1b43a00568dcfaf580e7082aadc12ebde5
Signed-off-by: Loic Poulain <loicx.poulain@intel.com>
Signed-off-by: Jian Luo <jian.luo@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 122149