Grease inside of slow cooker with butter or cooking spray. Combine filling ingredients and pour into slow cooker. Combine topping ingredients in a large mixing bowl until crumbly. Sprinkle over the rhubarb mixture. Set slow-cooker on low and cook for 2 1/2 hours. ***TIP: To make topping crispier, remove the slow-cooker lid 30 minutes before done. Place a piece of paper towel over the top of the slow-cooker and replace the cover. Cook for additional 30 minutes.