This appears to be similar to bug 706092.
I'm using Nightly 2012-08-24 (with the new markup panel)
1. go to a page
2. open the page inspector
3. open the markup panel
4. close the markup panel
5. close the page inspector
it closes, but there's a this.highlighter is null error in the web console. You can then open the page inspector again, but you won't be able to close it.
[09:21:52.724] TypeError: this.highlighter is null @ resource:///modules/inspector.jsm:1006
I also spotted this in the Error Console:
Timestamp: 8/24/12 9:45:38 AM
Error: TypeError: this._markupSplitter.parentNode is null
Source File: resource:///modules/inspector.jsm
Created attachment 655184 [details] [diff] [review]
Created attachment 655233 [details] [diff] [review]
patch to land
Moved fix for bug 785528 into that bug.