Steps to reproduce:
* Go to https://test.wikidata.org/wiki/Special:ApiSandbox#action=options&format=json&optionname=wikibase-entitytermsview-showEntitytermslistview&optionvalue=1&token=0c5f2f3b92d5d985ed91d4a462727d2c5729b96a%2B\
* Make sure you are logged in.
* Update the token parameter in the ApiSandbox.
* Click "make request".
Response headers I get:
```
Accept-Ranges: bytes
Age: 0
Backend-Timing: D=355897 t=1462352770925853
Cache-Control: no-cache
Content-Encoding: gzip
Content-Length: 20
Content-Type: text/html
Date: Wed, 04 May 2016 09:06:11 GMT
Server: mw1200.eqiad.wmnet
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Vary: Accept-Encoding
Via: 1.1 varnish, 1.1 varnish, 1.1 varnish
X-Cache: cp1068 pass+chfp(0), cp3041 pass+chfp(0), cp3042 frontend pass+chfp(0)
X-Firefox-Spdy: 3.1
X-Powered-By: HHVM/3.12.1
x-analytics: WMF-Last-Access=04-May-2016;https=1
x-client-ip: 87.138.110.76
x-content-type-options: nosniff
x-varnish: 198182218, 2132090503, 1656587994
```
But the response itself is missing. The option is not saved. You can check that with https://test.wikidata.org/w/api.php?action=query&format=jsonfm&meta=userinfo&uiprop=options.
Other usages of the same API work:
* Try to use an invalid token. You get the correct error message.
* Try an invalid option name. You get the correct error message.
* Try an other option, e.g. set `searchNs0` to true or false. This works just fine.
All this means that the issue is probably in `RepoHooks::onGetPreferences` in the Wikibase repo code base. But I can not reproduce it anywhere else, and test.wikidata.org does not tell me what's wrong. Is there an error log?
```Update: Got a stack trace.
```lang=json
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/htmlform/HTMLFormField.php","line":1098,"function":"setContext","class":"Message","type":"->","args":["NULL"]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/htmlform/HTMLFormField.php","line":384,"function":"getMessage","class":"HTMLFormField","type":"->","args":["string"]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/htmlform/HTMLForm.php","line":462,"function":"__construct","class":"HTMLFormField","type":"->","args":["array"]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiOptions.php","line":89,"function":"loadInputFromParameters","class":"HTMLForm","type":"::","args":["string","array"]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiMain.php","line":1367,"function":"execute","class":"ApiOptions","type":"->","args":[]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiMain.php","line":468,"function":"executeAction","class":"ApiMain","type":"->","args":[]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiMain.php","line":434,"function":"executeActionWithErrorHandling","class":"ApiMain","type":"->","args":[]},
{"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/htmlform/HTMLFormField.php","line":1098,"function":"setContext","class":"Message","type":"->","args":["NULL"]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/htmlform/HTMLFormField.php","line":384,"function":"getMessage","class":"HTMLFormField","type":"->","args":["string"]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/htmlform/HTMLForm.php","line":462,"function":"__construct","class":"HTMLFormField","type":"->","args":["array"]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiOptions.php","line":89,"function":"loadInputFromParameters","class":"HTMLForm","type":"::","args":["string","array"]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiMain.php","line":1367,"function":"execute","class":"ApiOptions","type":"->","args":[]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiMain.php","line":468,"function":"executeAction","class":"ApiMain","type":"->","args":[]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/includes/api/ApiMain.php","line":434,"function":"executeActionWithErrorHandling","class":"ApiMain","type":"->","args":[]}, {"file":"/srv/mediawiki/php-1.27.0-wmf.23/api.php","line":83,"function":"execute","class":"ApiMain","type":"->","args":[]}, {"file":"/srv/mediawiki/w/api.php","line":3,"function":"include","args":["string"]}
```