check for webviewcore before reading settings

In setActive(), the WebViewCore instance may
have already been destroyed before the settings
object can be read.

Change-Id: I46b2a57b2d92fc7216b1c984f7044fea1fc2409e
http://b/2570700

core/java/android/webkit/WebView.java

@@ -3974,7 +3974,7 @@ public class WebView extends AbsoluteLayout
                 // false for the first parameter
             }
         } else {
-            if (getSettings().getBuiltInZoomControls()
+            if (mWebViewCore != null && getSettings().getBuiltInZoomControls()
                     && !getZoomButtonsController().isVisible()) {
                 /*
                  * The zoom controls come in their own window, so our window