## Fixes an issue where the Signals count spinner can spin forever

Per the animated gif below, in `7.6` `BC 4`, the `Signals count` spinner on the Overview page spins forever until the signals index is created (in the current Kibana space):

![signals-count-loading-spinner](https://user-images.githubusercontent.com/4459398/73785251-2ca42000-4754-11ea-8671-daa81f351c9b.gif)

The `Signals count` spinner will spin forever until the user clicks the `Detections` tab, which-in turn creates the signals index (if it doesn't exist), per the animated gif below:

![create-signals-index](https://user-images.githubusercontent.com/4459398/73785319-4ba2b200-4754-11ea-9bb0-a745a8b2be5d.gif)

This behavior is an issue because:

- When a fresh deployment is created on Elastic Cloud, a user won't understand why the `Signals count` widget is always spinning on the `Overview` page. (The user must click the `Detections` page to resolve this.)
- In deployments where authentication is disabled, or, for _reasons_, a Detections index will never be created, the `Signals count` spinner on the Overview page will always spin.

To reproduce:

1. Spin up a new `7.6` `BC 4` deployment on Elastic Cloud
2. Login to Kibana for the first time
3. Navigate to the SIEM app

**Expected result**
- All histograms on the Overview page eventually stop displaying their respective loading spinners

**Actual result**
- The `Signals count` widget spinner spins forever. (The user must click the `Detections` tab to create the signals index.)

## Deleting the signals index

To reproduce the issue above when a signals index has already been created (by clicking on the Detections tab), run the following from the Kibana `Dev Tools` `Console`:

```
DELETE /.siem-signals-default-000001
```

It is also possible to reproduce this issue by creating a new space, because it won't have a signals index.

elastic/siem-team#514