I'm not sure the exact steps that are leading to this, but once I get in this state the error console spews:
Error: TypeError: this._panel.hidePopup is not a function
Source File: resource:///modules/devtools/DeveloperToolbar.jsm
*** Bug 784057 has been marked as a duplicate of this bug. ***
Bringing mratcliffe across from bug 784057 as the assignee (that bug is a dupe of this one and work had not begun yet)
Created attachment 654598 [details] [diff] [review]
A simple check to see if the node exists before removing it.