I created a small fix for this problem.
If you're experiencing a gui crash with MPS. (Clicking search)
Download this: http://ftbmonsterservers.com/Fixes/XACT v0.4.2c_shyfix.jar
Goto your Monster/mods folder. Drop that .jar in the mods folder and delete the old XACT_v0.4.2c.jar
file.
Load up Monster and it'll be resolved.
If you're experiencing a gui crash with MPS. (Clicking search)
Download this: http://ftbmonsterservers.com/Fixes/XACT v0.4.2c_shyfix.jar
Goto your Monster/mods folder. Drop that .jar in the mods folder and delete the old XACT_v0.4.2c.jar
file.
Load up Monster and it'll be resolved.