IE: Erros is thrown on New Page without focus

[Frederico Caldeira Knabben]

With IE8, I'm able to reproduce the problem with "exactly" these steps:

1. Load the replacebyclass sample.
2. Put the focus in another application with a small window, so you can still see the editor behind it. I use Skype for it.
3. Without clicking on anything else, click directly the New Page button in the editor toolbar.

A js error is thrown.

[Frederico Caldeira Knabben]

Additionally, it breaks only with compatibility mode enabled. No problem in IE8 mode.

[Garry Yao]

Can be reproduced without step2.

[Garry Yao]

Fixed with [5518].