Entire toolbar becomes disabled during Preview when using custom icon for Preview button

**Describe the bug**
When using a custom icon for the Preview button in the toolbar the entire set of toolbar icons becomes disabled, including the Preview toggle.

**To Reproduce**
Create a custom Preview button, like the one below. When the Preview button is clicked all buttons in the toolbar are disabled, including the Preview button, which makes it impossible to switch out of preview mode.

```
toolbar: [{
name: "preview",
action: EasyMDE.togglePreview,
className: "icon-preview",
title: "Preview",
}],
```

**Expected behavior**
All buttons _except_ the Preview, Side-By-Side and Full Screen view buttons should be disabled.