Lightly coat a 4- to 5-quart slow cooker with cooking spray; set aside. In a large skillet cook onion and garlic in hot oil over medium heat until tender, stirring occasionally. Stir in rice and salt. Cook for 1 minute, stirring occasionally.
Transfer rice mixture to the prepared cooker. Stir in broth, sweet peppers, the water, wine, thyme, and saffron.
Cover and cook on low-heat setting for 3 3/4 hours. Stir in corn and 1/2 cup cheese. Turn off cooker. Let stand, covered, for 10 minutes. If risotto is too dry, stir in a little water to reach desired consistency. If desired, sprinkle each serving with additional cheese.