Call to a member function structure() on null

Hi,

I suddenly get the error: Call to a member function structure() on null

Statamic\Structures\CollectionTree::Statamic\Structures{closure} vendor/statamic/cms/src/Structures/CollectionTree.php:19

return Blink::once('collection-tree-structure-'.$this->handle(), function () {

       ERROR====> return Collection::findByHandle($this->handle())->structure();

    });

Does someone know how to fix?