if (inv.Approval_Comment_Check__c == 'Requested')
{
  approvalStatements.put(inv.Id, inv);
  // Reset the field value to null,
  // so that the check is not repeated,
  // next time the object is updated
  inv.Approval_Comment_Check__c = null;
}

for (Invoice_Statement__c invs : [SELECT (SELECT ID
                                          FROM ProcessInstances
                                          ORDER BY CreatedDate DESC
                                          LIMIT 1)
                                  FROM Invoice_Statement__c
                                  WHERE ID IN :approvalStatements.keySet()])
{
    processInstanceIds.add(invs.ProcessInstances[0].Id);
}

// Now that we have the most recent process instances, we can check
// the most recent process steps for comments.
for (ProcessInstance pi : [SELECT TargetObjectId,
                               (SELECT Id, StepStatus, Comments
                                FROM Steps
                                ORDER BY CreatedDate DESC
                                LIMIT 1 )
                           FROM ProcessInstance
                           WHERE Id IN :processInstanceIds
                           ORDER BY CreatedDate DESC])
{
  // If no comment exists, then prevent the object from saving.
  if ((pi.Steps[0].Comments == null ||
       pi.Steps[0].Comments.trim().length() == 0))
  {
    approvalStatements.get(pi.TargetObjectId).addError(
     'Operation Cancelled: Please provide a reason ' +
     'for your approval / rejection!');
  }
}