Again, blank white pop-up window

Experiencing this problem in Opera browser and ONLY on one web-site! Had the same bug with 6.x-2.0-alfa4, then upgraded to 3.x-dev, nothing changed after that. As I said, this problem appear in Opera browser (v10.1) on one of my websites. On the other one with 6.x-2.0-alfa4 everything works fine even in Opera.

Opera's bug console says:

JavaScript - http://mysite.com/img_assist/load/textarea?textarea=body
Inline script thread
Error:
name: TypeError
message: Statement on line 4: Cannot convert undefined or null to Object
stacktrace: n/a; see opera:config#UserPrefs|Exceptions Have Stacktrace

WTF is that? This annoys me very much, 'coz Opera is my default browser and i hadn't anything like that before...