[Bug] Segmentation fault when adding contact by call, then deleting it and getting called again

Platform: X86_64
OS: Fedora 29
Build: based on f801d20f
Prerequisites: client A with client B as contact, client B with no contacts

Steps to reproduce:

1. In client A: Call client B
2. In client B: Reject call and accept trust request
3. In client B: Remove the newly added contact
4. In client A: Call client B

Expected behaviour: Client B shows the incoming call
Actual behaviour: Client B terminates with a segmentation fault