Kibana version: 6.4.0

Browser version: Firefox

Describe the bug:
If you change the index pattern in TSVB, the Time Field selector doesn't show any fields unless you refresh it by changing the Time Picker, even if there is data in the time interval already set. If you don't change the index pattern, there is no problem in populating the Time Field selector.

Steps to reproduce:

1. Create 2 index patterns ( .kibana and metricbeat-* in my case).
2. Set .kibana as the default index.
3. Open TSVB and go to Panel Options.
4. Change the index pattern from the default to metricbeat-*. Now it won't show any fields in the Time Fields selector.
5. Change the timepicker to another interval. Now the Time Fields selector will be populated with the date fields from the metricbeat-* index pattern.

It seems that the request to ES is not sent after the index pattern field is updated, even though it should be.