If you have updated to WordPress 5, please make sure to update your theme.
If you are using a version of a theme that was published before the release of WordPress 5, please note that it does not support WordPress 5, and needs to be updated. Basically, the version of the theme you are using was created when WordPress 5 did not exist, so the theme could not be tested and made compatible with it.
To use WordPress 5, please be sure to be running:
Storyteller 1.4 or newer
Bild 1.2 or newer
Essentialist 1.2 or newer
Stills 1.2 or newer
Narrator 1.2 or newer
To find out which version of a theme you are using, please go to Admin Panel > Appearance > Themes. Click the theme image, and see the version number near the theme name.
To update a theme, please go to your ThemeForest Downloads page and obtain the latest version. Then install it in the regular WordPress fashion (deactivate and remove the old theme, install the new one, install/update the required plugins).