Wrong typing of getPopupContainer

[zaycker]

By reviewing attachParent method: https://github.com/react-component/trigger/blob/89eefdb148ee68e4bbfe5b5b2887ceaff6b2e42c/src/index.tsx#L572-L590

that means getPopupContainer is able to return falsy value exactly "in case parent not ready". but it is blocked by strict typings: https://github.com/react-component/trigger/blob/89eefdb148ee68e4bbfe5b5b2887ceaff6b2e42c/src/index.tsx#L75

may I create PR? actually it is also existent in Tooltip where I came from initially...