在离子v3中突然我收到此错误，无法从未定义中读取_getPortal

Question

每当我调用ModalController或AlertController时，在ionic v3中都会出现此错误。似乎_appRoot未定义。有任何解决方法的想法吗？

core.js:1449 ERROR Error: Uncaught (in promise): TypeError: Cannot read property '_getPortal' of undefined
TypeError: Cannot read property '_getPortal' of undefined
    at App.present (app.js:241)
    at Alert.present (alert.js:101)
    at TaskDetailPage.<anonymous> (task-detail.ts:112)