[Payment Request] null checks _webState instance before calling a method on.

ios/chrome/browser/ui/payments/payment_request_manager.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/payments/payment_request_manager.h"
 
 #include "base/ios/block_types.h"
 #include "base/ios/ios_util.h"
 #import "base/mac/bind_objc_block.h"
 #include "base/mac/foundation_util.h"
@@ skipping 292 matching lines @@
       [self webState]->AddScriptCommandCallback(callback, kCommandPrefix);
 
       _webStateEnabled = YES;
     }
   } else {
     [self disableCurrentWebState];
   }
 }
 
 - (void)disableCurrentWebState {
-  if (_webStateEnabled) {
+  if (_webState && _webStateEnabled) {
     _webState->RemoveScriptCommandCallback(kCommandPrefix);
     _webStateEnabled = NO;
   }
 }
 
 - (void)disconnectWebState {
   if (_webState) {
     _paymentRequestJsManager = nil;
     _webStateObserver.reset();
     [self disableCurrentWebState];
@@ skipping 458 matching lines @@
 #pragma mark - CRWWebStateObserver methods
 
 - (void)webState:(web::WebState*)webState
     didCommitNavigationWithDetails:
         (const web::LoadCommittedDetails&)load_details {
   [self dismissUI];
   [self enableCurrentWebState];
 }
 
 @end