Dialer, Contacts and Messenger apps closing automatically in Oneplus X
I had one plus 1 back in 2015 and it was really good but with age it started showing similar issues. I took it to the service center and they said the problem is with OS and they are going to format and reinstall. 

Luckily i got the upgraded OS but it didn't really help me, after 1 month i started facing the same problem then i visited the service center again and finally they end up replacing my handset to a new one.