Fix the issue that when topic is invalid, the callback was not triggered.

Example/Messaging/Tests/FIRMessagingServiceTest.m

@@ -269,6 +269,34 @@ - (void)testUnsubscribeCompletionHandlerWithSuccess {
                                }];
 }
 
+- (void)testSubscriptionCompletionHandlerWithInvalidTopicName {
+  XCTestExpectation *subscriptionCompletionExpectation =
+      [self expectationWithDescription:@"Subscription is complete"];
+  [_messaging subscribeToTopic:@"!@#$%^&*()"
+                    completion:^(NSError *_Nullable error) {
+                      XCTAssertNotNil(error);
+                      XCTAssertEqual(error.code, FIRMessagingErrorInvalidTopicName);
+                      [subscriptionCompletionExpectation fulfill];
+                    }];
+  [self waitForExpectationsWithTimeout:0.2
+                               handler:^(NSError *_Nullable error){
+                               }];
+}
+
+- (void)testUnsubscribeCompletionHandlerWithInvalidTopicName {
+  XCTestExpectation *unsubscriptionCompletionExpectation =
+      [self expectationWithDescription:@"Unsubscription is complete"];
+  [_messaging unsubscribeFromTopic:@"!@#$%^&*()"
+                        completion:^(NSError *error) {
+                          XCTAssertNotNil(error);
+                          XCTAssertEqual(error.code, FIRMessagingErrorInvalidTopicName);
+                          [unsubscriptionCompletionExpectation fulfill];
+                        }];
+  [self waitForExpectationsWithTimeout:0.2
+                               handler:^(NSError *_Nullable error){
+                               }];
+}
+
 - (void)testFIRMessagingSDKVersionInFIRMessagingService {
   Class versionClass = NSClassFromString(kFIRMessagingSDKClassString);
   SEL versionSelector = NSSelectorFromString(kFIRMessagingSDKVersionSelectorString);

Firebase/Messaging/FIRMessaging.m

@@ -760,6 +760,9 @@ - (void)subscribeToTopic:(NSString *)topic
   }
   FIRMessagingLoggerError(kFIRMessagingMessageCodeMessaging009,
                           @"Cannot parse topic name %@. Will not subscribe.", topic);
+  if (completion) {
+    completion([NSError fcm_errorWithCode:FIRMessagingErrorInvalidTopicName userInfo:nil]);
+  }
 }
 
 - (void)unsubscribeFromTopic:(NSString *)topic {
@@ -786,6 +789,9 @@ - (void)unsubscribeFromTopic:(NSString *)topic
   }
   FIRMessagingLoggerError(kFIRMessagingMessageCodeMessaging011,
                           @"Cannot parse topic name %@. Will not unsubscribe.", topic);
+  if (completion) {
+    completion([NSError fcm_errorWithCode:FIRMessagingErrorInvalidTopicName userInfo:nil]);
+  }
 }
 
 #pragma mark - Send

Firebase/Messaging/FIRMessagingPubSub.h

@@ -147,6 +147,7 @@ NS_ASSUME_NONNULL_BEGIN
  */
 
 + (NSString *)removePrefixFromTopic:(NSString *)topic;
+
 /**
  *  Check if the topic name has "/topics/" prefix.
  *

Firebase/Messaging/Public/FIRMessaging.h

@@ -189,6 +189,10 @@ typedef NS_ENUM(NSUInteger, FIRMessagingError) {
 
   /// Some parameters of the request were invalid.
   FIRMessagingErrorInvalidRequest = 7,
+
+  /// Topic name is invalid for subscription/unsubscription.
+  FIRMessagingErrorInvalidTopicName = 8,
+
 } NS_SWIFT_NAME(MessagingError);
 
 /// Status for the downstream message received by the app.