Do not call onDrop if newIndex is null

src/lib/sortable-hoc.jsx

@@ -26,8 +26,10 @@ const SortableHOC = function (WrappedComponent) {
                     return a.top - b.top;
                 });
             } else if (!newProps.dragInfo.dragging && this.props.dragInfo.dragging) {
-                this.props.onDrop(Object.assign({},
-                    this.props.dragInfo, {newIndex: this.getMouseOverIndex()}));
+                const newIndex = this.getMouseOverIndex();
+                if (newIndex !== null) {
+                    this.props.onDrop(Object.assign({}, this.props.dragInfo, {newIndex}));
+                }
             }
         }