[ios] Active web state observer in tab collection.

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

Index: ios/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.mm
diff --git a/ios/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.mm b/ios/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.mm
index 4f1be95a138fbede477a10838c6df09292275a00..2bc0022405217b032abcd9a24dff53a3b4fa5290 100644
--- a/ios/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.mm
+++ b/ios/clean/chrome/browser/ui/find_in_page/find_in_page_mediator.mm
@@ -72,9 +72,11 @@
 
 - (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);

[edchin, 2017/05/27 18:47:26]

This crashes when the last web state is detached while the tab container is
shown. 
So I added the nil check above. Does stopFinding need to be called at the time
the web state is detached?

+    DCHECK(helper);
+    helper->StopFinding(nil);
+  }
 }
 
 - (void)findResultsAvailable:(FindInPageModel*)model {