Add a null check in StartSurfaceLayout#shrinkTab().

Bug: 1091507
Change-Id: I29a291ee7a9db36a665bf0663c27f08b876486ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406684Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806653}

chrome/android/features/start_surface/internal/java/src/org/chromium/chrome/features/start_surface/StartSurfaceLayout.java

@@ -200,7 +200,7 @@ public class StartSurfaceLayout extends Layout implements StartSurface.OverviewM
             return;
         }
 
-        shrinkTab(() -> mTabListDelegate.getThumbnailLocationOfCurrentTab(false));
+        shrinkTab(animate, () -> mTabListDelegate.getThumbnailLocationOfCurrentTab(false));
     }
 
     @Override
@@ -290,11 +290,16 @@ public class StartSurfaceLayout extends Layout implements StartSurface.OverviewM
 
     /**
      * Animate shrinking a tab to a target {@link Rect} area.
+     * @param animate Whether to play an entry animation.
      * @param target The target {@link Rect} area.
      */
-    private void shrinkTab(Supplier<Rect> target) {
-        forceAnimationToFinish();
+    private void shrinkTab(boolean animate, Supplier<Rect> target) {
+        if (target.get() == null) {
+            mController.showOverview(animate);
+            return;
+        }
 
+        forceAnimationToFinish();
         LayoutTab sourceLayoutTab = mLayoutTabs[0];
         CompositorAnimationHandler handler = getAnimationHandler();
         Collection<Animator> animationList = new ArrayList<>(5);