Retain code verification promise.

Signal/src/ViewControllers/Registration/CodeVerificationViewController.m

@@ -281,7 +281,7 @@ NS_ASSUME_NONNULL_BEGIN
     [self startActivityIndicator];
     OWSProdInfo([OWSAnalyticsEvents registrationRegisteringCode]);
     __weak CodeVerificationViewController *weakSelf = self;
-    [self.accountManager registerWithVerificationCode:[self validationCodeFromTextField] pin:nil]
+    [[self.accountManager registerWithVerificationCode:[self validationCodeFromTextField] pin:nil]
             .then(^{
                 OWSProdInfo([OWSAnalyticsEvents registrationRegisteringSubmittedCode]);
 
@@ -313,7 +313,7 @@ NS_ASSUME_NONNULL_BEGIN
                         [weakSelf.challengeTextField becomeFirstResponder];
                     }
                 });
-        });
+            }) retainUntilComplete];
 }
 
 - (void)verificationWasCompleted