fix: 🐛 get children or parent delay (#1641)

packages/x6/src/model/cell.ts

@@ -750,30 +750,26 @@ export class Cell<
     return this.store.get('parent')
   }
 
-  getParent() {
-    let parent = this._parent
-    if (parent == null && this.store) {
-      const parentId = this.getParentId()
-      if (parentId != null && this.model) {
-        parent = this.model.getCell(parentId)
-        this._parent = parent
-      }
+  getParent<T extends Cell = Cell>(): T | null {
+    const parentId = this.getParentId()
+    if (parentId && this.model) {
+      const parent = this.model.getCell<T>(parentId)
+      this._parent = parent
+      return parent
     }
-    return parent
+    return null
   }
 
   getChildren() {
-    let children = this._children
-    if (children == null) {
-      const childrenIds = this.store.get('children')
-      if (childrenIds && childrenIds.length && this.model) {
-        children = childrenIds
-          .map((id) => this.model?.getCell(id))
-          .filter((cell) => cell != null) as Cell[]
-        this._children = children
-      }
-    }
-    return children ? [...children] : null
+    const childrenIds = this.store.get('children')
+    if (childrenIds && childrenIds.length && this.model) {
+      const children = childrenIds
+        .map((id) => this.model?.getCell(id))
+        .filter((cell) => cell != null) as Cell[]
+      this._children = children
+      return [...children]
+    }
+    return null
   }
 
   hasParent() {