Add nullcheck for VelocityTracker

Bug:134712476
Change-Id:I7834507a4e9aadf4d89c0ed6f3eba812df7425cd

src/com/android/launcher3/dragndrop/FlingToDeleteHelper.java

@@ -106,6 +106,7 @@ public class FlingToDeleteHelper {
      * @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);
         }