Adding removed null check in BrowsingModeBottomToolbarCoordinator

Bug: 1055466, 1054072
Change-Id: I36d21fc5d28c4144b788e204f33b4f66fdae0b33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066585
Commit-Queue: Filip Gorski <fgorski@chromium.org>
Auto-Submit: Filip Gorski <fgorski@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744079}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/bottom/BrowsingModeBottomToolbarCoordinator.java

@@ -143,6 +143,7 @@ public class BrowsingModeBottomToolbarCoordinator {
         mTabProvider.addObserver(new Callback<Tab>() {
             @Override
             public void onResult(Tab tab) {
+                if (tab == null) return;
                 TabImpl tabImpl = (TabImpl) tab;
                 final Tracker tracker = TrackerFactory.getTrackerForProfile(tabImpl.getProfile());
                 final Runnable completeRunnable = () -> {