The new Good Lock is amazing and brings much needed customization to your Galaxy, and the best part, it brings back the old way of using Multi Window. However, if you are unable to use some apps from Good Lock, you can sideload the apk from SamMobile's APK archive. It worked for me and now I can get Task Changer running which did not work for me earlier. https://www.sammobile.com/apk/
