Reproduce step is.
1. Site should be 3.80
2. Create some customer without password.
3. Upgrade you site to 3.90
4. Edit those customer who having null password entry in CustomerPassword Table.
5. Try to change password you will get error. Because you didn't handle null condition in below line.
return customerPassword.Password.Equals(savedPassword);
On PasswordsMatch method.
We did below change for solution:
if(customerPassword.Password == null)
return false;
return customerPassword.Password.Equals(savedPassword);
Thanks,
Jatin