Method `isPreviewActive()` is not working properly

**Describe the bug**
The `isPreviewActive()` method is returning `false`, whether the preview mode is active or not.

**Steps to reproduce the behavior:**
```html

Example / Preview

const easyMDE = new EasyMDE({
toolbar: [
{
className: 'fa fa-eye',
default: true,
name: 'preview',
noDisable: true,
title: 'Toggle Preview',
action: handleTogglePreview
},
'|',
'heading', 'bold', 'italic', 'strikethrough',
'|', 'quote', 'code',
'|', 'unordered-list', 'ordered-list',
'|', 'clean-block',
'|', 'link', 'image',
'|', 'table'
],
});

function handleTogglePreview() {
easyMDE.togglePreview();

// easyMDE.isPreviewActive() is always false.
if (easyMDE.isPreviewActive()) {
console.log('Preview is active.');
} else {
console.log('Preview is NOT active.');
}
}

```

1. Copy the code above into an HTML file (update paths to `easymde.min.js` and `easymde.min.css` accordingly)
1. Open the HTML file in a browser
1. Click the `Toggle preview` button
1. `easyMDE.isPreviewActive()` returns `false`
1. Click the `Toggle preview` button
1. `easyMDE.isPreviewActive()` returns `false` again

**Expected behavior**
`easyMDE.isPreviewActive()` should return `true` when preview mode is active. Else, `false`.

**Version information**
- OS: Ubuntu 20.04.1 LTS
- Browser: 89.0.4389.114
- EasyMDE version: 2.15.0 (latest)

**Additional context**
- https://github.com/Ionaru/easy-markdown-editor/blob/dbab7e95e7777d353b1600729709f01b876565f9/src/js/easymde.js#L2859
- `easyNDE.isSideBySideActive()` has the same issue.

Start in src/js/easymde.js around line 2859 and reproduce the issue with the provided HTML example in a browser. Check the related isSideBySideActive() behavior as well. Done means isPreviewActive() returns true while preview mode is active and false after it is toggled off.

Written by the indexing model from the issue text.