[IE]: Background color panel remains visible when another toolbar button is clicked.

[James]

Steps to reproduce the defect:

1. Open the Ajax sample in IE.
2. Click on the background color icon.
3. Notice that the following error:

Webpage error details

Message: 'null' is null or not an object
Line: 46
Char: 1279
Code: 0
URI: http://nightly.ckeditor.com/6288/ckeditor.js

3. Click on the numbered list icon.

Expected: The background color panel disappears and a numbered list is created.

Actual: An error has occurred on the page. The background color panel remains visible and a numbered list is created.

[Garry Yao]

Fixed with [6292].