Prevent toolbar toggle when checkSelection() with `disableToolbar` option enabled

With this config
```js
new MediumEditor(".editable",{
	disableToolbar: true
})
````

When you select the text it throws:
```
Uncaught TypeError: Cannot read property 'style' of undefined medium-editor.js:227
MediumEditor.checkSelection medium-editor.js:227
(anonymous function)
````