Fix NPE when connect timeout

If user click connect button and then receive timeout event after leave
detail page, the NPE happened. Ignore the timeout event if activity
already gone.

Bug: 131141866
Test: manual test
Test: make RunSettingsRoboTests -j32 ROBOTEST_FILTER=com.android.settings.wifi.details.WifiDetailPreferenceControllerTest
Change-Id: Ic22c22027386d4a6de5d693eafcd9cd463ed3415

src/com/android/settings/wifi/details/WifiDetailPreferenceController.java

@@ -1109,6 +1109,10 @@ public class WifiDetailPreferenceController extends AbstractPreferenceController
             }
             @Override
             public void onFinish() {
+                if (mFragment == null || mFragment.getActivity() == null) {
+                    Log.d(TAG, "Ignore timeout since activity not exist!");
+                    return;
+                }
                 Log.e(TAG, "Timeout for state:" + mConnectingState);
                 if (mConnectingState == STATE_ENABLE_WIFI) {
                     updateConnectingState(STATE_ENABLE_WIFI_FAILED);