This build is just to fix a bug that caused an error when previewing in lots of projects in r186. In case you missed it, r186 had lots of other changes - view the r186 release notes here to find out more.
This is a good time to remind beta users that occasional hiccups like this are to be expected on the beta channel. While we appreciate all testing feedback on beta releases, if you want to avoid disruption we recommend sticking to stable releases. Also as ever you should keep regular backups, especially before updates, in case of software or hardware failures. Construct 2 has built-in features to make automatic backups so make sure that's set up if you haven't done so already.