CBAS index creation on meta().id causes bucket insights panel to fail to load

Steps to reproduce:

1. Load travel-sample bucket
2. Create Dataset with:

CREATE DATASET testing ON `travel-sample` WHERE `callsign` IS NOT NULL

3. Create the following index:

CREATE INDEX testing_index ON testing(meta().id)

At this stage, the bucket insights panel fails to refresh, though I am still able to retrieve results from the Dataset.

Restarting the CBAS process and Couchbase Server did not fix the issue. The only solution was to drop the index, at which stage the Bucket Insights panel refreshes immediately.

I could not identify anything seemingly of interest in the analytics logs.

I see the following in ns_server debug logs:

[menelaus:warn,2018-12-31T07:17:21.441-08:00,ns_1@127.0.0.1:<0.4097.0>:menelaus_web:log_client_error:805]Client-side error-report for user "Administrator" on node 'ns_1@127.0.0.1':

User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36

Failed to get bucket insights.

Try refreshing the bucket insights.