[cloud_firestore] Fixes crash on Android when run a transaction

packages/cloud_firestore/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.12.9+5
+
+* Fixes a crash on Android when running a transaction without an internet connection.
+
 ## 0.12.9+4
 
 * Fix integer conversion warnings on iOS.

packages/cloud_firestore/android/src/main/java/io/flutter/plugins/firebase/cloudfirestore/CloudFirestorePlugin.java

@@ -373,10 +373,10 @@ public void onMethodCall(MethodCall call, final Result result) {
           final Map<String, Object> arguments = call.arguments();
           getFirestore(arguments)
               .runTransaction(
-                  new Transaction.Function<Map<String, Object>>() {
+                  new Transaction.Function<TransactionResult>() {
                     @Nullable
                     @Override
-                    public Map<String, Object> apply(@NonNull Transaction transaction) {
+                    public TransactionResult apply(@NonNull Transaction transaction) {
                       // Store transaction.
                       int transactionId = (Integer) arguments.get("transactionId");
                       transactions.append(transactionId, transaction);
@@ -424,23 +424,31 @@ public void notImplemented() {
                             Tasks.await(transactionTCSTask, timeout, TimeUnit.MILLISECONDS);
 
                         // Once transaction completes return the result to the Dart side.
-                        return transactionResult;
+                        return new TransactionResult(transactionResult);
                       } catch (Exception e) {
                         Log.e(TAG, e.getMessage(), e);
-                        result.error("Error performing transaction", e.getMessage(), null);
+                        return new TransactionResult(e);
                       }
-                      return null;
                     }
                   })
               .addOnCompleteListener(
-                  new OnCompleteListener<Map<String, Object>>() {
+                  new OnCompleteListener<TransactionResult>() {
                     @Override
-                    public void onComplete(Task<Map<String, Object>> task) {
-                      if (task.isSuccessful()) {
-                        result.success(task.getResult());
-                      } else {
+                    public void onComplete(Task<TransactionResult> task) {
+                      if (!task.isSuccessful()) {
                         result.error(
                             "Error performing transaction", task.getException().getMessage(), null);
+                        return;
+                      }
+
+                      TransactionResult transactionResult = task.getResult();
+                      if (transactionResult.exception == null) {
+                        result.success(transactionResult.result);
+                      } else {
+                        result.error(
+                            "Error performing transaction",
+                            transactionResult.exception.getMessage(),
+                            null);
                       }
                     }
                   });
@@ -820,6 +828,21 @@ public void onFailure(@NonNull Exception e) {
         }
     }
   }
+
+  private static final class TransactionResult {
+    final @Nullable Map<String, Object> result;
+    final @Nullable Exception exception;
+
+    TransactionResult(@NonNull Exception exception) {
+      this.exception = exception;
+      this.result = null;
+    }
+
+    TransactionResult(@Nullable Map<String, Object> result) {
+      this.result = result;
+      this.exception = null;
+    }
+  }
 }
 
 final class FirestoreMessageCodec extends StandardMessageCodec {