Custom toolbars can't be placed below the editor

Placing the toolbar below the editor results in errors:

```
Uncaught TypeError: Cannot read property 'querySelectorAll' of null
at s.resetDialogInputs (trix.js:17)
at new s (trix.js:17)
at new a (trix.js:20)
at HTMLElement.connect (trix.js:21)
at HTMLElement.n (trix.js:15)
```

##### Steps to Reproduce

```

```

##### Details

* Trix version: 1.0.0
* Browser name and version: Chrome 71
* Operating system: Mac OS 10.14

Start with the toolbar connection path shown in the trix.js stack trace, especially resetDialogInputs and HTMLElement.connect. Reproduce the issue using the supplied markup with the trix-editor before the trix-toolbar, then verify that the editor initializes without the querySelectorAll error when the toolbar is below it.

Written by the indexing model from the issue text.