Check nullity in FullscreenHtmlApiHandler

chrome/android/java/src/org/chromium/chrome/browser/fullscreen/FullscreenHtmlApiHandler.java

Index: chrome/android/java/src/org/chromium/chrome/browser/fullscreen/FullscreenHtmlApiHandler.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/fullscreen/FullscreenHtmlApiHandler.java b/chrome/android/java/src/org/chromium/chrome/browser/fullscreen/FullscreenHtmlApiHandler.java
index 344496951d0dcf50dcb8e0061ec91f1b6e71a7ca..66a6faed9dee468cc671bc3f0d95dc2fe6c527c3 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/fullscreen/FullscreenHtmlApiHandler.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/fullscreen/FullscreenHtmlApiHandler.java
@@ -270,7 +270,12 @@ public class FullscreenHtmlApiHandler {
             }
         };
         contentView.addOnLayoutChangeListener(mFullscreenOnLayoutChangeListener);
-        contentViewCore.getWebContents().exitFullscreen();
+
+        // getWebContents() will return null if contentViewCore has been destroyed
+        if (contentViewCore.getWebContents() != null) {
+            contentViewCore.getWebContents().exitFullscreen();
+        }
+
         if (mFullscreenInfoBarDelegate != null) {
             mFullscreenInfoBarDelegate.closeFullscreenInfoBar();
             mFullscreenInfoBarDelegate = null;