Discover breaks when index pattern gets deleted

[flash1293]

Kibana version: master, 7.7.0

Elasticsearch version: master, 7.7.0

Describe the bug: When discover is accessed and the last used index pattern is not available anymore, an empty page gets rendered.

Steps to reproduce:

1. Go to discover, change the index pattern
2. Go to management, delete the index pattern you selected
3. Go back to discover
4. Empty screen

Expected behavior:

Should switch to the default index pattern and show a toast.

Any additional context:

I haven't tested but this definitely used to work at some point, there is code in place to handle it: https://github.com/elastic/kibana/blob/master/src/legacy/core_plugins/kibana/public/discover/np_ready/angular/discover.js#L140

I suspect some call is throwing an expecting and preventing the execution of this code path (maybe https://github.com/elastic/kibana/blob/master/src/legacy/core_plugins/kibana/public/discover/np_ready/angular/discover.js#L138).

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kertal]

this definitely worked a a while ago, thanks, will take care of it