Fix draftjs type error for event

Summary:
Fixes an issue introduced by D13137413 where we weren't safely getting the right document for an event target, if the event target is null.

Fixes by using the safer `getCorretDocumentFromNode`, which uses the defualt `document` if the argument is null.

Reviewed By: claudiopro

Differential Revision: D18618225

fbshipit-source-id: bc381a5000318c5065fdf856387dd038b7e04003

Author: Jack Armstrong

src/component/handlers/drag/DraftEditorDragHandler.js

@@ -20,6 +20,7 @@ const EditorState = require('EditorState');
 const ReactDOM = require('ReactDOM');
 
 const findAncestorOffsetKey = require('findAncestorOffsetKey');
+const getCorrectDocumentFromNode = require('getCorrectDocumentFromNode');
 const getTextContentFromFiles = require('getTextContentFromFiles');
 const getUpdatedSelectionState = require('getUpdatedSelectionState');
 const getWindowForNode = require('getWindowForNode');
@@ -36,13 +37,12 @@ function getSelectionForEvent(
   let node: ?Node = null;
   let offset: ?number = null;
 
+  const eventTargetDocument = getCorrectDocumentFromNode(event.currentTarget);
   /* $FlowFixMe(>=0.68.0 site=www,mobile) This comment suppresses an error
    * found when Flow v0.68 was deployed. To see the error delete this comment
    * and run Flow. */
-  const {ownerDocument} = event.currentTarget;
-
-  if (typeof ownerDocument.caretRangeFromPoint === 'function') {
-    const dropRange = ownerDocument.caretRangeFromPoint(event.x, event.y);
+  if (typeof eventTargetDocument.caretRangeFromPoint === 'function') {
+    const dropRange = eventTargetDocument.caretRangeFromPoint(event.x, event.y);
     node = dropRange.startContainer;
     offset = dropRange.startOffset;
   } else if (event.rangeParent) {