Hi there. I'm having the same problem. My macbook pro did an Imovie upgrade on Oct 30th. Now Imovie 10.1.3 crashes immediately. If you want to follow the solution given here, do this:
1. Click on finder (lower left corner, the little square face)
2. On the upper side of the screen click "GO"
3. You are going to get a drop down menu. Press and HOLD the "alt-option" key on the bottom left side of your keyboard. The word "Library" is going to appear on the "GO" drop down menu. The word "Library" will "only" appear when you press on the "alt-option" key on your keyboard. Keep pressing the "alt-option" key and click on "Library" on the drop down menu.
4. You are going to get a screen with folders, look for the folder named "Containers" and Click on it.
5. After clicking on "Containers" You are going to get another screen with folders, look for the one named "Com.Apple.Imovie app" This is the folder "to my understanding" has to be deleted. By dragging it to the trash bin icon on the lower right corner of the screen. You may go a step forward by emptying the trash bin and deleting the file for good.
I haven't deleted the "com.apple.Imovie app" folder yet, because it contains projects and videos I had on Imovie. I'm not sure if I will lose them, when I restart Imovie again. Maybe someone can tell us that on this forum.