I found answer - "It probably means that the "Post Processing" field at the bottom of the Export panel is blank (default should be 'Do Nothing'). This has cropped up a few times since the LR4 beta started. Simply enter a valid option and the error should disappear." by Jim01403 and it did work. Thanks!
To cool... I had the same issue. I was getting ready to rebuild my system. Then thought, ok try google. I like good ole Google!! then I found your responce. Thank you Drajs!!