Check GetVisibleItem exists in GetFormattedURL

When Clearing Presented state, it is possible that no Visible item
exists.
Check before dereferencing.

Fix cl/2606873002

BUG=674986

Review-Url: https://codereview.chromium.org/2608733003
Cr-Commit-Position: refs/heads/master@{#440957}

ios/chrome/browser/ui/toolbar/toolbar_model_impl_ios.mm

@@ -84,6 +84,11 @@ bool ToolbarModelImplIOS::ShouldDisplayHintText() {
 base::string16 ToolbarModelImplIOS::GetFormattedURL(size_t* prefix_end) const {
   base::string16 formatted_url = toolbar_model_->GetFormattedURL(prefix_end);
   Tab* current_tab = delegate_->GetCurrentTab();
+  if (!current_tab || !current_tab.webState ||
+      !current_tab.webState->GetNavigationManager() ||
+      !current_tab.webState->GetNavigationManager()->GetVisibleItem()) {
+    return formatted_url;
+  }
   GURL url =
       current_tab.webState->GetNavigationManager()->GetVisibleItem()->GetURL();
   if (reading_list::IsOfflineURL(url) &&