Fix code for NPE when click add and cancel repeatly VPN profile

Symptom: NPE is happened when click add and cancel repeatly VPN profile

Root Cause: Exception handling for ConfigDialog object

Steps to reproduce :
1. Go to VPN menu in Settings without Secure LockScreen
2. Click add VPN profile button
3. Keyguard dialog show
4. Click cancel button
5. Repeat 2 - 4 step

Test: refer a "Steps to reproduce"

Solution: Add exception handling for ConfigDialog object. This is a very rare case. But exception handling code should be needed.

Signed-off-by: SangJin Cha <sj.cha@lge.com>
Change-Id: I4c680b383f94c86a1c806d986e56f9f00d2be10e