fix(drag): error when drop selection text to outside

affects: @he-tree/vue

Author: phphe

packages/he-tree-vue/src/components/DraggableTree.ts

@@ -250,7 +250,7 @@ const cpt = defineComponent({
  const rootEl = this.getRootEl();
  let dragElement: HTMLElement | null = null; // dragElement is the drag node element
  const removePlaceholderWhenEnd = () => {
- if (targetTree!.has(targetTree!.placeholderData)) {
+ if (targetTree?.has(targetTree!.placeholderData)) {
  targetTree!.ignoreUpdate(() => {
  targetTree!.remove(targetTree!.getStat(targetTree!.placeholderData));
  // update together to prevent flick