Java script error when click on Toggle Full Screen Mode

Steps to reproduce

Precondition
Open Fire bug on FF
On Edit View
Open a page contains XHTML property
Click on XHTML to enable Tiny MCE
Click on Toggle Full Screen Mode button more than 2 times
OBSERVER
In Console tab of Fire bug will show js error:

TypeError: this.controls is null
http://vnlabvm02:17004/to/UI/CMS/7.6.4.0/ClientResources/Editor/tiny_mce/tiny_mce_src.js
Line 14714
TypeError: ed.selection is null
http://vnlabvm02:17004/to/UI/CMS/7.6.4.0/ClientResources/Editor/tiny_mce/plugins/epicontentfragment/editor_plugin_src.js
Line 65