add null checker

Author: argzdev

firebase-installations/src/main/java/com/google/firebase/installations/FirebaseInstallations.java

@@ -422,6 +422,8 @@ private void doNetworkCallIfNecessary(boolean forceRefresh) {
   private synchronized void updateFidListener(
       PersistedInstallationEntry prefs, PersistedInstallationEntry updatedPrefs) {
     if (fidListeners.size() != 0
+        && prefs.getFirebaseInstallationId() != null
+        && updatedPrefs.getFirebaseInstallationId() != null
         && !prefs.getFirebaseInstallationId().equals(updatedPrefs.getFirebaseInstallationId())) {
       // Update all the registered FidListener about fid changes.
       for (FidListener listener : fidListeners) {