After moving gatekeeper.pattern.key to unlock, Settings>Security keeps crashing
In a web browser (on any device), log into Gmail with the account you used when setting up your phone.
Go to https://myaccount.google.com/intro/find-your-phone
Press "Lock your phone"
Enter "abcd"
Your phone's screen is now locked. Unlock it with "abcd"
You can now change your screen lock.