[iOS] Fix crash when closing last tab with FindInPage active

The reason for the crash is that when the last tab is closed, there
is no more current web state, so it's impossible to get a FindTabHelper.

Bug: 1044606
Change-Id: I1f47ca7751c0f808e6d53abd4cb8356dd68de126
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023831Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#735844}

ios/chrome/browser/ui/find_bar/find_bar_coordinator.mm

@@ -66,10 +66,16 @@
   if (!self.presenter.isPresenting) {
     return;
   }
-  FindTabHelper* helper = FindTabHelper::FromWebState(self.currentWebState);
+
   // If the FindUI is still active, the dismiss should be unanimated, because
   // the UI will be brought back later.
-  BOOL animated = helper && !helper->IsFindUIActive();
+  BOOL animated;
+  if (self.currentWebState) {
+    FindTabHelper* helper = FindTabHelper::FromWebState(self.currentWebState);
+    animated = helper && !helper->IsFindUIActive();
+  } else {
+    animated = true;
+  }
   [self.findBarController findBarViewWillHide];
   [self.presenter dismissAnimated:animated];
 }