Dynamic Workflow error

I've created new workflow that works just like Solution Starter Dynamic Workflow ( I need to have two instances of the same wf). Problem is, in new workflow, when I click on "Submit", workflow breaks.

Error is in the following function:

protected override System.Workflow.ComponentModel.ActivityExecutionStatus Execute(System.Workflow.ComponentModel.ActivityExecutionContext executionContext)
        {
            var psiService = executionContext.GetService<IProxyPSIService>();
            Approvers = psiService.GetGroupMembers(
                ApprovalInfo.GroupsUids,
                ProjectWorkflowContext,
                ApprovalInfo.FilterByDepartment);

            Approvers.AddRange(
                psiService.GetUsers(
                    ApprovalInfo.UsersUids,
                    ProjectWorkflowContext,
                    ApprovalInfo.FilterByDepartment));
            Approvers = Approvers.Distinct().ToList();

            InitEmailService(executionContext);
            return base.Execute(executionContext);
        }

executionContext.GetService<IProxyPSIService>() returns null.

I haven't defined any of Service References in project (because I don't know what to define, I can not open Service Reference in old workflow).

Any idea what could be the problem?