Dragging a structure item into a visual markdown field crashes codemirror

[rasteiner]

Hi!

dragging (while sorting) a structure item into the visual markdown editor crashes the javascript (it seems to be coming from codemirror, but having the files minimized it's difficult to tell).

This issue alone isn't really critical, I can tell my clients "well, don't drag them there, silly!".

But it becomes critical when this plugin is used together with the "tabs" plugin, which effectively puts all tabs on top of each other.

So I'm crossposting this issue because I don't really know where it belongs. The detailed issue description is there.