Check for null overview mode behavior in NTPLayout.initialize()

R=yusufo@chromium.org

Bug: 990979
Change-Id: Ie198810daeef73289d3ff559735a079b236a1cf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755415Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Matt Simmons <mattsimmons@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688162}

chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageLayout.java

@@ -260,7 +260,7 @@ public class NewTabPageLayout extends LinearLayout implements TileGroup.Observer
         if (VrModuleProvider.getDelegate().isInVr()) onEnterVr();
 
         mOverviewModeBehavior = overviewModeBehavior;
-        if (overviewModeBehavior.overviewVisible()) {
+        if (overviewModeBehavior != null && overviewModeBehavior.overviewVisible()) {
             mOverviewObserver = new EmptyOverviewModeObserver() {
                 @Override
                 public void onOverviewModeFinishedHiding() {