Prompt background does not disappear after 2nd try

```
When clicking the image button, the wmd-prompt-background appears. If the image
insert is canceled using callback(null); in JavaScript, then the background
disappears. However, if the user clicks the image button again, the
wmd-prompt-background will appear again, but whether callback(null); or
callback("http://someurlhere.com/photo.jpg"); is used, the background will not
disappear.

The JavaScript Console shows at this time "Uncaught TypeError: Cannot call
method 'removeChild' of null" in Markdown.Editor.js line 1684.

I have attached the trace that the browser's console provides. The browser
version is Google Chrome 20.0.1132.57 m on Windows XP.
```

Original issue reported on code.google.com by `johngti...@gmail.com` on 25 Jul 2012 at 11:06

Attachments:
- [trace.txt](https://storage.googleapis.com/google-code-attachments/pagedown/issue-31/comment-0/trace.txt)