Ensure Client exists (#3900)

* Fix 238932261

* Change log

Author: wu-hui

firebase-firestore/CHANGELOG.md

@@ -3,6 +3,7 @@ by opting into a release at
 [go/firebase-android-release](http:go/firebase-android-release) (Googlers only).
 
 # Unreleased
+- [fixed] Fixed an issue `waitForPendingWrites()` could lead to NullPointerException.
 
 # 24.2.0
 - [feature] Added `TransactionOptions` to control how many times a transaction

firebase-firestore/src/main/java/com/google/firebase/firestore/FirebaseFirestore.java

@@ -550,6 +550,7 @@ public Task<Void> terminate() {
    */
   @NonNull
   public Task<Void> waitForPendingWrites() {
+    ensureClientConfigured();
     return client.waitForPendingWrites();
   }