I had a customer with this issue and I tried to just close out of the texting app by double clicking the home button and closing the whole messaging application out. After that did not work I tried to do a complete reset (settings -> general -> reset -> erase all content and settings) to the phone and wiping it clean and even with it being wiped and nothing being on the phone the application still didnt work at all. Then I did a straight restore (different form of doing a reset on the phone done by holding the home button down and plugging the phone to the computer) . So I ordered a new phone for the customer and it came brand new in the box. When I turned it on and went and tested the emoji application it worked fine. After doing a back up and restore on the phone the emoji started doing the same thing when it came back on after restoring from a back up. I then did everything I did with the first phone on the NEW phone and with nothing on it tried to go to the emoji's and it AGAIN kept closing out. This is a software glitch of some sort and can ONLY be fixed by an update. I've been a phone tech for 2 years so I have a really good round about technical issues on phones and on androids and apple products. Most of the time an update always fixes the "little" (big to some people) issues.
🙂Hope this helps a little bit!