Trebuchet: fix NPE during touch events

E/AndroidRuntime(  693): FATAL EXCEPTION: main
E/AndroidRuntime(  693): java.lang.NullPointerException: Attempt to invoke
                         virtual method 'int android.view.View.getMeasuredWidth()'
                         on a null object reference
E/AndroidRuntime(  693):    at com.android.launcher3.PagedView.onTouchEvent
                            (PagedView.java:1986)

Change-Id: Ica9395a1e2b2d28a1b074b40ff759eab68154ff0
(cherry picked from commit 16416d46)