If you're running the game via a web browser, then try to use the standalone SWF player (
Flash Player Projector) instead.
More directly: forget the in-game dialogue editor. Export your work to an external TXT file (via the
Save button). Edit it using a proper editing application (with features such as Undo, Find-Replace, Spellcheck, etc). And then use the
Load button to bring your completed file back into the game for playtesting.