Stir together chicken stock, Great Northern beans, thyme, salt, pepper, garlic, celery, carrots, and onion in a 5- to 6-quart slow cooker. Place ham bone in the center of mixture; cover and cook on HIGH until beans are tender, about 6 hours. Remove ham bone; let stand until cool enough to handle. Remove meat from bone; discard fat, gristle, and bone. Shred meat, and stir into soup.