Additional work on concurrent dom modification

Sortable.js

@@ -707,7 +707,7 @@
 
 					if (rootEl !== parentEl) {
 						newIndex = _index(dragEl);
-
+						if (newIndex != -1) {
 							// drag from one list and drop into another
 							_dispatchEvent(null, parentEl, 'sort', dragEl, rootEl, oldIndex, newIndex);
 							_dispatchEvent(this, rootEl, 'sort', dragEl, rootEl, oldIndex, newIndex);
@@ -718,6 +718,7 @@
 							// Remove event
 							_dispatchEvent(this, rootEl, 'remove', dragEl, rootEl, oldIndex, newIndex);
 						}
+					}
 					else {
 						// Remove clone
 						cloneEl && cloneEl.parentNode.removeChild(cloneEl);
@@ -725,12 +726,13 @@
 						if (dragEl.nextSibling !== nextEl) {
 							// Get the index of the dragged element within its parent
 							newIndex = _index(dragEl);
-
+							if (newIndex != -1) {
 								// drag & drop within the same list
 								_dispatchEvent(this, rootEl, 'update', dragEl, rootEl, oldIndex, newIndex);
 								_dispatchEvent(this, rootEl, 'sort', dragEl, rootEl, oldIndex, newIndex);
 							}
 						}
+					}
 
 					if (Sortable.active) {
 						// Drag end event
@@ -1092,6 +1094,9 @@
 	 * @private
 	 */
 	function _index(/**HTMLElement*/el) {
+		if (!el || !el.parentNode) {
+			return -1;
+		}
 		var index = 0;
 		while (el && (el = el.previousElementSibling)) {
 			if (el.nodeName.toUpperCase() !== 'TEMPLATE') {
@@ -1151,10 +1156,8 @@
 		index: _index
 	};
 
-
 	Sortable.version = '1.2.2';
 
-
 	/**
 	 * Create sortable instance
 	 * @param {HTMLElement}  el