I came across this error on a site I was helping to troubleshoot today and with a little Googling found it happens when you move the DOM containing a TinyMCE editor to another place in the document: TinyMCE loses it’s reference to that original element.

To get around this, showing or hiding the editor won’t work, instead destroying the element before you move it and recreating it (same as the previous post) does the trick:

tinyMCE.execCommand('mceRemoveControl', false, 'idOfElement');
// move DOM element
tinyMCE.execCommand('mceAddControl', false, 'idOfElement');