Fix NPE on network forget from details page.

Forgetting a network from the wifi network details page throws an NPE in
WifiSettings when the details fragment exits, crashing the app.

Bug: b/63351286
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: I749ecbda5654ff7a86ffe4c38ca74ebe338923f0
2 files changed