Fix crash when trying to add a reviewer during 'Serial document review' workflow

Caused by: javax.faces.el.EvaluationException: [Error: WorkflowVariables["participants"][(int)WorkflowVariables.index]: array index out of bounds.]
[Near :

{... Unknown ....}]
^
[Line: 1, Column: 0]
at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
... 102 more
Caused by: [Error: WorkflowVariables["participants"][(int)WorkflowVariables.index]: array index out of bounds.]
[Near : {... Unknown ....}

]
^