Update PopUpManager.as

hi, I found "root setter" function has a issue.
...
var popUp:DisplayObject = popUps[i];
var overlay:DisplayObject = DisplayObject(POPUP_TO_OVERLAY[i]);
...

the overlay always return null, I think because 'POPUP_TO_OVERLAY[i]' should be 'POPUP_TO_OVERLAY[popUp]';
Text Editors: visibility fixes when text is empty (closes #693)
GroupedList: fixed null reference on item renderer maps when changing item renderers (closes #688)
Item renderers: accessory is cleared if itemHasAccessory is set to false (closes #689)
Merge pull request #692 from lynxerzhang/patch-1

Fixed issue where changing PopUpManager root failed to transfer overlays.
Merge branch 'master' of github.com:joshtynjala/feathers
Drawers: can touch docked drawers (in addition to the content) to close an open drawer (closes #685)
Drawers: simplified last commit, hitArea is used as a fallback if no children are touched (similar to Scroller)
List, GroupedList: selection cleared on data provider change, similar to scroll position being reset to 0,0 (closes #695)
List, GroupedList: updated docs to warn about scroll position resets and selection clearing on data provider change
GroupedList: updateItemAt() with a single index will properly update all renderers in a group
ComponentsExplorer: fixed typos
BitmapFontTextRenderer: uses QuadBatch's batchable property instead of switching between a QuadBatch and separate images. Where the batchable QuadBatch replaces separate images, BitmapFontTextRenderer will perform significantly better with longer text (references #629)
BitmapFontTextRenderer: avoid calls to push()
Scale9Image: uses QuadBatch's batchable property instead of switching between a QuadBatch and separate images. Where the batchable QuadBatch replaces separate images, Scale9Image will perform better (references #629)
Scale3Image: uses QuadBatch's batchable property instead of switching between a QuadBatch and separate images. Where the batchable QuadBatch replaces separate images, Scale3Image will perform better (references #629)
Scale9Image, Scale3Image: removed unnecessary code
TiledImage: added useSeparateBatch property, which controls QuadBatch's batchable (closes #629)
DragAndDrop: new example (closes #580)
DefaultGroupedListHeaderOrFooterRenderer: if contentLabel is used, its maxWidth is set to allow word wrap