Fixed race condition in Spinner.DialogPopup on null reference.

NullPointerException is triggered when DialogPopup is
dismissed due to object mPopup being null when not showing.
Race condition is triggered when dialog is dismissed before
touch event to dismiss dialog has been processed properly.

Fixed by adding a null check prior to accessing mPopup object.

Change-Id: I3c4fe9c279d92a7103fb4f68189d1d42beedc606