Resolve touch bar crash issue when default search provider is disabled.

Default search provider maybe null when it's disabled by policy. Make
sure the touch bar search button can handle this case with an empty button.
User is still able to press the button to enter URL.

Screenshot: https://drive.google.com/file/d/1Sw8xd2bl4MGfF2p2qrT7PKF4bQ2jczRC/view?usp=sharing

Bug: 1011260
Change-Id: Iba221a4d65baff7cc5485de43f4b145481c2d1f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841837
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709125}

chrome/browser/ui/cocoa/touchbar/browser_window_default_touch_bar.mm

@@ -485,14 +485,17 @@ class API_AVAILABLE(macos(10.12.2)) TouchBarNotificationBridge
       TemplateURLServiceFactory::GetForProfile(browser_->profile());
   const TemplateURL* defaultProvider =
       templateUrlService->GetDefaultSearchProvider();
-  BOOL isGoogle =
-      defaultProvider->GetEngineType(templateUrlService->search_terms_data()) ==
-      SEARCH_ENGINE_GOOGLE;
-
-  base::string16 title =
-      isGoogle ? l10n_util::GetStringUTF16(IDS_TOUCH_BAR_GOOGLE_SEARCH)
-               : l10n_util::GetStringFUTF16(IDS_TOUCH_BAR_SEARCH,
-                                            defaultProvider->short_name());
+  BOOL isGoogle = NO;
+  base::string16 title;
+  if (defaultProvider) {
+    isGoogle =
+        defaultProvider->GetEngineType(
+            templateUrlService->search_terms_data()) == SEARCH_ENGINE_GOOGLE;
+
+    title = isGoogle ? l10n_util::GetStringUTF16(IDS_TOUCH_BAR_GOOGLE_SEARCH)
+                     : l10n_util::GetStringFUTF16(
+                           IDS_TOUCH_BAR_SEARCH, defaultProvider->short_name());
+  }
 
   NSImage* image = nil;
 #if BUILDFLAG(GOOGLE_CHROME_BRANDING)