If you are using Messages - here's what worked for me. Go to > Settings > Applications (Apps or Application Manager, depending on your phone) > go to the app 'Messages' > open and if it says Default ...
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If you are using Messages - here's what worked for me. Go to > Settings > Applications (Apps or Application Manager, depending on your phone) > go to the app 'Messages' > open and if it says Default app, tap in that and take default off. Then, again, go to > Settings > search 'Default' and tap on Default Applications > Messaging > tap on Messages to make default again and everything should work fine. If necessary, restart phone. This is the only thing that worked for me when I started getting messages to change default messaging app to the Samsung one (out of nowhere) and clicked 'No', then I couldn't type anymore in Messages.