advice from Apple Care
I have been having the same problem and spoke today with a rep from Apple Care. Here's what he told me. Quit iMovie; go to Computer > Movies and move the iMovie Projects folder to the desktop. Open iMovie; create a new test project using film footage from the Event Library. Try to export it or to do whatever you were doing when it crashed (selecting Titles below a certain row in the Titles window made mine crash as well as exporting). If you do these things and iMovie does not crash, the problem is being caused by a corrupted movie project. (Although the rep wouldn't say so, iMovie is causing the corrupting--and the problem lies with the titles--more on this below.) Just having a corrupted project in the Project Library will make the program crash, even if you are working on a new project. You have to rebuild your file piece by piece, seeing if it exports after each addition to see if the new addition is causing the problem (choose tiny movie because it's quicker). I managed to retrieve all the edited video plus transitions by clicking on a film clip in the corrupted project and selecting all and copying it and importing into a new project (though in between, I had to open and close iMovie to drag that corrupted project back to the desktop because it kept crashing). I discovered that one particular kind of title--Organic Main-- was making the project and program crash--not all of the titles. When I tried to add it back to the rebuilt project, it crashed; when I went to the old file that was corrupted and tried to delete that title, it crashed. When I tried to delete the clip that had that title on it in the old project, it crashed. The transitions I used worked fine (though not sure if all of them would). Music, photos, film clips, etc. all fine. The problem lies with iMovie and it needs a fix! But I was able to rebuild my project--minus the style of title I like--and export it using this method.
