Img bb-code: there is empty field in pop-up window when users try to add tag to selected text

When I try to add tag "Insert Picture" to selected text there is empty field in pop-up window.

• Steps to reproduce*

1. Open forum on Create Topic page
2. Write some text, for example "SelectedText"
3. Select this text
4. Press "Insert picture"

• AR: URL link is empty.
• ER: URL link cossists of selected text.

Note: if I add text into URL link and press "OK", final tag is consists of my added text and selected text from test-case.