Error: System.DmlException: Merge failed. First exception on row 0 with id first error: DELETE_FAILED,TargetX_SRMb.ContactTrigger: execution of AfterDelete caused by: System.NullPointerException: Attempt to de-reference a null object Trigger.TargetX_SRMb.ContactTrigger: line 188, column 1: 
Trigger.TargetX_SRMb.ContactTrigger is as part of Managed package that has been installed. In sandbox, I'm running the test class for the batch class in developer console using system Admin profile.
Why do I get this error? How can I fix it?