Fix race in credentialsProvider.setChangeListener (#919)

Author: schmidt-sebastian

firebase-firestore/CHANGELOG.md

@@ -9,6 +9,8 @@
 - [feature] Added an `addSnapshotsInSyncListener()` method to 
   `FirebaseFirestore`that notifies you when all your snapshot listeners are
   in sync with each other.
+- [fixed] Fix a race condition that could cause a `NullPointerException` during
+  client initialization.
 
 # 21.1.2
 - [fixed] Fixed a crash that could occur when a large number of documents were

firebase-firestore/src/main/java/com/google/firebase/firestore/core/FirestoreClient.java

@@ -96,19 +96,6 @@ public FirestoreClient(
 
     TaskCompletionSource<User> firstUser = new TaskCompletionSource<>();
     final AtomicBoolean initialized = new AtomicBoolean(false);
-    credentialsProvider.setChangeListener(
-        (User user) -> {
-          if (initialized.compareAndSet(false, true)) {
-            hardAssert(!firstUser.getTask().isComplete(), "Already fulfilled first user task");
-            firstUser.setResult(user);
-          } else {
-            asyncQueue.enqueueAndForget(
-                () -> {
-                  Logger.debug(LOG_TAG, "Credential changed. Current user: %s", user.getUid());
-                  syncEngine.handleCredentialChange(user);
-                });
-          }
-        });
 
     // Defer initialization until we get the current user from the changeListener. This is
     // guaranteed to be synchronously dispatched onto our worker queue, so we will be initialized
@@ -127,6 +114,21 @@ public FirestoreClient(
             throw new RuntimeException(e);
           }
         });
+
+    credentialsProvider.setChangeListener(
+        (User user) -> {
+          if (initialized.compareAndSet(false, true)) {
+            hardAssert(!firstUser.getTask().isComplete(), "Already fulfilled first user task");
+            firstUser.setResult(user);
+          } else {
+            asyncQueue.enqueueAndForget(
+                () -> {
+                  hardAssert(syncEngine != null, "SyncEngine not yet initialized");
+                  Logger.debug(LOG_TAG, "Credential changed. Current user: %s", user.getUid());
+                  syncEngine.handleCredentialChange(user);
+                });
+          }
+        });
   }
 
   public Task<Void> disableNetwork() {