can’t show embed form

All Core modules and non-core modules are latest version.

And I did 'composer update' and version of all packages ares latest.

In this condition, when I tried to show create-edit-page which contains embed form, Internal Server Error occurs.

Three weeks ago, I made this page and it could show with no error at that time.

I never touch code of embed form within three weeks.

Current error is like below:

[2018-08-17 15:02:40] local.ERROR: Object of class StolzAssetsManager could not be converted to string (View: /dev/resources/themes/admin/layouts/embed.blade.php) {"userId":1,"email":"[email protected]","exception":"[object] (ErrorException(code: 0): Object of class Stolz\Assets\Manager could not be converted to string (View: /dev/resources/themes/admin/layouts/embed.blade.php) at /dev/storage/framework/views/10ad0517f8e4982888df65e7774f707a133435d2.php:6, ErrorException(code: 0): Object of class Stolz\Assets\Manager could not be converted to string at /dev/storage/framework/views/10ad0517f8e4982888df65e7774f707a133435d2.php:6)

[stacktrace]

I don't know the reason to ocuur this error.