NPE check in VpnSettings.

Bug: 3368545

Also fix a couple of bugs in TetherSettings:

Bug: 3090156 - view was being reused without removing from old dialog
Bug: 3368493 - preference being removed a second time

Change-Id: I8fe513d41e1f3af6994783be5ead69b6bd205711

src/com/android/settings/vpn/VpnSettings.java

@@ -725,6 +725,8 @@ public class VpnSettings extends SettingsPreferenceFragment
     }
 
     private void startVpnEditor(final VpnProfile profile, boolean add) {
+        if (getActivity() == null) return;
+
         Bundle args = new Bundle();
         args.putParcelable(KEY_VPN_PROFILE, profile);
         // TODO: Show different titles for add and edit.