If you get the error:

Undefined field _text_

Please make sure to open up solrconfig.xml in your Opensolr Control Panel Admin UI and remove the reference to the _text_ field under the /update initParams:

<initparams path="/update/**,/query,/select,/tvrh,/elevate,/spell,/browse">
 <lst name="defaults"></lst>
 <str name="df">_text_</str>
</initparams>