Process signal not working for some times

Hi
i have camunda process it have service JDEJobServiceTaskHandler it will call external service once complete need to give signal and continue rest of the process.
@Component
public class JDEJobServiceTaskHandler extends AbstractBpmnActivityBehavior {
@Override
public void execute(ActivityExecution execution) throws Exception {

}
@Override
public void signal(ActivityExecution execution, String signalName, Object signalData) throws Exception {
leave(execution);
}
}

after some activity calling following line from some other class
runtimeService.signal(executionId, processVariableMap);
some times giving signal and working fine, but some time getting following exception and not completing process.

getting follwing Exception:
org.camunda.bpm.engine.BadUserRequestException: execution b50e0f08-1f9b-11e9-b70b-c2c5379d7c5a doesn’t exist: execution is null

process not ending. manually killing.

Thanks
Venkatesan.A