Fix Crash CarrierDefaultApp due to exception

[Issue]
CarrierDefaultApp crashes due to null pointer exception.

[Cause of defect]
To call mUrl.toString() even if mUrl is null and mUrl is already
checked if it is null just before.

[How to fix]
Check the null status of mUrl when call its toString() method.

Bug: 65393337

Change-Id: I4b4b1f125aac7e30ca18be22d7adc358cab83b2f