Fix ConnectivityManager.requestNetwork crash issue when try again

- When the user requests "try again", the Network-Request-Dialog will be
  recreated. If the Wi-Fi framework sends the callback result at this
  time, the Network-Request-Dialog may not be ready at the moment,
  causing the NPE to crash.

- Add NPE checks to avoid crash issues.

Bug: 220679437
Test: manual test
make RunSettingsRoboTests \
  ROBOTEST_FILTER=NetworkRequestDialogActivityTest

Change-Id: I8e48a29649e72d9f8c67c4ac24f0eb27ba600dae