The "add a caption" button in the captions panel is currently broken on beta/local.
It doesn't add a captions row, but instead shows this error in console:
```
Uncaught TypeError: Cannot read property 'filter' of undefined
at CaptionsPanel.createIndexedEditableRow (load.php?lang=en&modules=ext.uls.common%2Ccompactlinks%2Cinterface|jquery%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows|jquery.uls|mediawiki.ui.button%2Cicon|mmv.bootstrap|oojs-ui-core.styles|wikibase.mediainfo.filePageDisplay%2Cstatements&skin=vector&version=18qbuys:559)
at CaptionsPanel.addNewEditableLanguageRow (load.php?lang=en&modules=ext.uls.common%2Ccompactlinks%2Cinterface|jquery%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows|jquery.uls|mediawiki.ui.button%2Cicon|mmv.bootstrap|oojs-ui-core.styles|wikibase.mediainfo.filePageDisplay%2Cstatements&skin=vector&version=18qbuys:569)
at load.php?lang=en&modules=ext.uls.common%2Ccompactlinks%2Cinterface|jquery%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows|jquery.uls|mediawiki.ui.button%2Cicon|mmv.bootstrap|oojs-ui-core.styles|wikibase.mediainfo.filePageDisplay%2Cstatements&skin=vector&version=18qbuys:549
at OoUiButtonWidget.oo.EventEmitter.emit (<anonymous>:274:486)
at OoUiButtonWidget.OO.ui.mixin.ButtonElement.onClick (load.php?lang=en&modules=ext.uls.common%2Ccompactlinks%2Cinterface|jquery%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows|jquery.uls|mediawiki.ui.button%2Cicon|mmv.bootstrap|oojs-ui-core.styles|wikibase.mediainfo.filePageDisplay%2Cstatements&skin=vector&version=18qbuys:203)
at HTMLAnchorElement.dispatch (load.php?lang=en&modules=ext.uls.common%2Ccompactlinks%2Cinterface|jquery%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows|jquery.uls|mediawiki.ui.button%2Cicon|mmv.bootstrap|oojs-ui-core.styles|wikibase.mediainfo.filePageDisplay%2Cstatements&skin=vector&version=18qbuys:94)
at HTMLAnchorElement.elemData.handle (load.php?lang=en&modules=ext.uls.common%2Ccompactlinks%2Cinterface|jquery%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows|jquery.uls|mediawiki.ui.button%2Cicon|mmv.bootstrap|oojs-ui-core.styles|wikibase.mediainfo.filePageDisplay%2Cstatements&skin=vector&version=18qbuys:90)
```
**Acceptance criteria:**
On https://commons.wikimedia.beta.wmflabs.org/wiki/File:Photo_on_08-02-2019_at_16.35.jpg
When captions panel is in edit mode:
[] "Add a caption" should add a captions row (without errors in console)