In some cases, if you're installing a theme or extension over a Magento instance that previously had a different theme, or already has categories created, you may experience this exception on the frontend.
Usually, to solve it, a reindex of the catalog is required. You can do that by using the following command:
php bin/magento indexer:reindex
More thorough information about the reindexing process can be found in this article:
How to reindex your Magento 2 store from the Admin or via SSH
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