Fix crash with g_variant_unref

Fix crash with g_variant_unref

- With 'enable' method, g_dbus_proxy_call_sync() returns GVariant
  and g_variant_get() deconstructs that GVariant instance.
- It causes below error and sometimes crashes.
  [g_variant_unref: assertion 'value != NULL' failed]
- Testing with another method, issue was not found with g_variant_unref()
- Patch newly uploaded due to merge conflict.
  (Previous patch : http://165.213.149.170/gerrit/#/c/151298/)

Change-Id: I4c8b7aadcd96a6e5b86f6136e85a1e7ed48588ae
Signed-off-by: Jiwan Kim <ji-wan.kim@samsung.com>