Fix Menu Button null pointer

When exiting the tab switcher the Activity Tab can be null. This CL
adds a null check to safeguard against this.

Bug: 934116
Change-Id: I2a0fdead0faf43a05dfe25e483664e5a2164f291
Reviewed-on: https://chromium-review.googlesource.com/c/1480701Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634752}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/HomeButton.java

@@ -105,7 +105,6 @@ public class HomeButton extends ChromeImageButton
         mActivityTabTabObserver = new ActivityTabTabObserver(activityTabProvider) {
             @Override
             public void onObservingDifferentTab(Tab tab) {
-                if (tab == null) return;
                 updateButtonEnabledState();
             }
 
@@ -126,6 +125,10 @@ public class HomeButton extends ChromeImageButton
 
     private boolean isActiveTabNTP() {
         if (mActivityTabProvider == null) return false;
-        return NewTabPage.isNTPUrl(mActivityTabProvider.getActivityTab().getUrl());
+
+        final Tab tab = mActivityTabProvider.getActivityTab();
+        if (tab == null) return false;
+
+        return NewTabPage.isNTPUrl(tab.getUrl());
     }
 }