修复拖拽棋子报错

assets/script/logic/gridMap/UEGridMap.ts

@@ -129,7 +129,7 @@ export default class UEGridMap extends UEBase {
     }
 
     private OnTouchMove(event: cc.Event.EventTouch): void {
-        if (!this.selectedCell) return;
+        if (!this.selectedCell || !this.dragStartPos) return;
 
         const touchPos = this.gridLayer.convertToNodeSpaceAR(event.getLocation());
         const distance = touchPos.sub(this.dragStartPos).mag();