Open image dialog on existing element cause error

[Garry Yao]

Environment

3.1.x branch

Reproducing Procedures

1. Open 'replacebyclass' sample page and load the document with the following source:

   <p>
   <img alt="" src="http://dev.fckeditor.net/chrome/site/logos.gif" /></p>
   

2. click on the image and open 'Image' dialog;

• Actual Result: JavaScript exception thrown.

[Garry Yao]

Regression of [4685].
Ticket test of that is telling the same thing:

​http://ckeditor.t/tt/4246/2.html

[Garry Yao]

We need to make sure the preview is well setup before setup dialog contents.

[Garry Yao]

Correction, should be regression of [4665].

[Garry Yao]

Fixed with [4787].