Settings: Fix crash on WifiAdvancedSetting

When back to WifiAdvancedSetting from "WLAN Direc"
or "Install certificates",Settings app will crash
due to try to remove a null preference.

We should check if the preference is null before
remove it.

Change-Id: I7d67b44b0391679e509890bf5aa4b30cad1e5258
CRs-Fixed: 1059802