Add nullcheck for VelocityTracker

Bug:134712476
Change-Id:I7834507a4e9aadf4d89c0ed6f3eba812df7425cd
(cherry picked from commit 6ef5b8abf47fb6782bb1274bca26cf9739d4389f)
diff --git a/src/com/android/launcher3/dragndrop/FlingToDeleteHelper.java b/src/com/android/launcher3/dragndrop/FlingToDeleteHelper.java
index 589ad25..06b5c40 100644
--- a/src/com/android/launcher3/dragndrop/FlingToDeleteHelper.java
+++ b/src/com/android/launcher3/dragndrop/FlingToDeleteHelper.java
@@ -106,6 +106,7 @@
      * @return the vector at which the item was flung, or null if no fling was detected.
      */
     private PointF isFlingingToDelete() {
+        if (mVelocityTracker == null) return null;
         if (mDropTarget == null) {
             mDropTarget = (ButtonDropTarget) mLauncher.findViewById(R.id.delete_target_text);
         }