If you're running 8.5, it's recommended that you upgrade to one of the patch releases. A number of issues were corrected with widgets in 8.5.1, but 8.5.5 is due to be released today, so I would wait for that one.
Also, you may want to test under a different browser to see if the behavior is consistent.
If you still have problems, please provide us with the mashup and other entities for recreating the issue, and we will take a closer look.
I work with Juliana.
Answering your question, unfortunately, no error message is displayed in the logs (ApplicationLog, ScriptLog, ScriptErrorLog, etc.), or in the Developer tools browser. I tried to reproduce the error, while tracking the logs (using tail), but I didn't identify anything.
When doing a fresh install of 8.5+, we're not seeing the beta widgets. Did you perform an upgrade from an earlier major/minor release?
We don't see Beta widgets in the Mashup editor either, but the "Enable Beta Features" checkbox is still available on the PlatformSubsystem, as of 8.5.4. I was just wondering what it does (if anything).
The decision was made to leave the toggle for this feature in place for future use. It currently does not provide any functionality.
Beta widgets are Themeable ones: http://support.ptc.com/help/thingworx_hc/thingworx_8_hc/en/index.html#page/ThingWorx/Help/Mashup_Bui...
Hope it helps,
If I understand it correctly, themable widgets are out of beta in 8.5, but the option is still there as of 8.5.4. Looks like there's something else...
Did you confirm exactly which patch level of ThingWorx 8.5 you are running? If you're still on 8.5.0, it is best to upgrade due to the many bug fixes that are included in the latest releases. Once you have done so, please re-assess for the problem.
Something else that could be causing the issue is the existence of custom extensions or old Marketplace extensions that are not compatible with 8.5.
I recommend opening a case for your issue. Please let me know if you would prefer that I open it for you.