requesting preview of folder results in error 500

[individual-it]

Steps to reproduce

1. curl '172.17.0.1/owncloud-core/remote.php/dav/files/admin/folder1/?x=320&y=320&forceIcon=1&preview=1' -u admin:admin -v

Expected behaviour

good error message

Actual behaviour

*   Trying 172.17.0.1...
* TCP_NODELAY set
* Connected to 172.17.0.1 (172.17.0.1) port 80 (#0)
* Server auth using Basic with user 'admin'
> GET /owncloud-core/remote.php/dav/files/admin/folder1/?x=320&y=320&forceIcon=1&preview=1 HTTP/1.1
> Host: 172.17.0.1
> Authorization: Basic YWRtaW46YWRtaW4=
> User-Agent: curl/7.58.0
> Accept: */*
> 
< HTTP/1.1 500 Internal Server Error
< Date: Wed, 25 Mar 2020 08:40:58 GMT
< Server: Apache/2.4.41 (Ubuntu)
< X-Content-Type-Options: nosniff
< X-XSS-Protection: 1; mode=block
< X-Robots-Tag: none
< X-Frame-Options: SAMEORIGIN
< X-Download-Options: noopen
< X-Permitted-Cross-Domain-Policies: none
< Set-Cookie: oc4kyqahfanl=gmb9m8a7hmimnt37ftfoto96fp; path=/owncloud-core; HttpOnly
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Cache-Control: no-store, no-cache, must-revalidate
< Pragma: no-cache
< Set-Cookie: oc_sessionPassphrase=ASqpnB51kYMxo6%2FCksoeppsAbEKkFpyTPmZfrwrqQweB9L3cpevK2dYGjP34O%2BS9T2vbMDBkgK9VVD6TXUmlasfbYHGNoYjLw7AO5HnLT%2B5cF7lKXA34BAnglSC69o3T; path=/owncloud-core; HttpOnly
< Content-Security-Policy: default-src 'none';
< Set-Cookie: oc4kyqahfanl=pdo1ffmfqblbk76eb4vr7hghkq; path=/owncloud-core; HttpOnly
< Set-Cookie: cookie_test=test; expires=Wed, 25-Mar-2020 09:40:58 GMT; Max-Age=3600
< Content-Length: 0
< Connection: close
< Content-Type: text/html; charset=UTF-8
< 
* Closing connection 0

[phil-davis]

Or a request for a preview/thumbnail of a folder could return a preview image that is a folder icon.
(and to be really fancy, return a folder icon image that has inside it a mini-preview of the first file in the folder)