"Platform map --> Validate" causes normal editor to hang

[pjw]

Just what the subject says. You can open a platform map with the editor while in normal "Canyon" mode. If you try to validate, the editor will lock up completely, and you have to shut down the game to get back to normal functionality.

This should, instead, result in a message. ("You must be in platform mode to validate a platform map." or somesuch.)