[ios] Active web state observer in tab collection.

ios/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.mm

 // Copyright 2017 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/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/scoped_observer.h"
 #import "ios/chrome/browser/find_in_page/find_in_page_model.h"
 #import "ios/chrome/browser/find_in_page/find_tab_helper.h"
@@ skipping 54 matching lines @@
     _scopedWebStateListObserver = base::MakeUnique<
         ScopedObserver<WebStateList, WebStateListObserverBridge>>(
         _webStateListObserver.get());
     _scopedWebStateListObserver->Add(_webStateList);
   }
   return self;
 }
 
 - (void)stopFinding {
   web::WebState* webState = self.webStateList->GetActiveWebState();
-  FindTabHelper* helper = FindTabHelper::FromWebState(webState);
-  DCHECK(helper);
-  helper->StopFinding(nil);
+  if (webState) {
+    FindTabHelper* helper = FindTabHelper::FromWebState(webState);
+    DCHECK(helper);
+    helper->StopFinding(nil);
+  }
 }
 
 - (void)findResultsAvailable:(FindInPageModel*)model {
   id<FindInPageConsumer> consumer = self.provider.consumer;
   [consumer setCurrentMatch:model.currentIndex ofTotalMatches:model.matches];
 }
 
 - (void)webStateList:(WebStateList*)webStateList
     didChangeActiveWebState:(web::WebState*)newWebState
                 oldWebState:(web::WebState*)oldWebState
@@ skipping 39 matching lines @@
 - (void)findPreviousInPage {
   web::WebState* webState = self.webStateList->GetActiveWebState();
   FindTabHelper* helper = FindTabHelper::FromWebState(webState);
   DCHECK(helper);
   helper->ContinueFinding(FindTabHelper::REVERSE, ^(FindInPageModel* model) {
     [self findResultsAvailable:model];
   });
 }
 
 @end