Fixes NullPointerException when url is null and user is navigating back

from an error page in a custom tab.

Bug: 1287952
Change-Id: Ie61dd911792405f10bb75f346db74f0f257f1f95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3398638
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Alex Mitra <alexmitra@chromium.org>
Cr-Commit-Position: refs/heads/main@{#960879}

chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabDelegateFactory.java

@@ -471,7 +471,7 @@ public ContextMenuPopulatorFactory createContextMenuPopulatorFactory(Tab tab) {
     public NativePage createNativePage(String url, NativePage candidatePage, Tab tab) {
         // Navigation comes from user pressing "Back to safety" on an interstitial so close the tab.
         // See crbug.com/1270695
-        if (url.equals(UrlConstants.NTP_URL) && tab.isShowingErrorPage()) {
+        if (UrlConstants.NTP_URL.equals(url) && tab.isShowingErrorPage()) {
             mNavigationDelegate.closeTab();
         }
         // Custom tab does not create native pages.