[Daniel Sattelmeyer]

Recently been having this problem with FCP 7 and the following worked for me to fix this KGCore Plugin error from crashing my FCP:

Right or control click on your sequences and export them as XMLs. Then, make sure they’ve exported correctly- I’d try importing them into a new project and making sure it imports correctly. If you can open them in a new project, do one of two things:

1) Use this new project as your starting point, importing all your XMLs into this new project, along with all your other media (footage, audio, graphics, etc.)

2) If you’d prefer not to reorganize yourself in a new project, go back to your old project and delete all the sequences you have exported XMLs for. Then simply reimport those XMLs and you should be up and running again, minus the KG Core plugin error.

This works for me. Just remember to keep a hearty autosave going, along with manual saving as often as you can. Apple+S is your friend.