The first weird behaviour is that this error message cannot be closed (Mac OS X latest edition) and KNIME has to be force quit for it to disappear.
The cause of it is however that the node's state is green instead of red and the Interactive view can be accessed.
It is also expected that an error window pops up on script errors. The script entered in the node's dialog is essentially wrapped in a try/catch block, which issues an alert window in case of errors.
What is strange though, is that you can't close this window. Could you provide a screenshot of the described behavior? Is it because the window is too large for the screen? Are the buttons missing on the bottom?