Thank you gbalakri and jlo.

I have added a verification of null for procins before accesing the [0] element of it and also I have made statusToUpdate as a parameter so we can send it as a String from a Process Builder for example.
 

public class CaseApprovalRecall {
    @InvocableMethod
    public static void recallCase(List<RecallCaseRequest> requests){
        if(requests != null && requests.size() > 0){
            RecallCaseRequest request = requests[0];
            Approval.ProcessWorkItemRequest pwr = new Approval.ProcessWorkItemRequest();
            List<ProcessInstance> procins = new List<ProcessInstance>([select Id from ProcessInstance where Status = 'Pending' and TargetObjectId = :request.caseId]);
            if((procins != null) && (procins.size() > 0)){
                // Retrieve the process work instance id associated to the process instance
                List<ProcessInstanceWorkitem>  workitem = new List<ProcessInstanceWorkitem>([select Id from ProcessInstanceWorkitem where ProcessInstanceId = :procins[0].id]);
                if ((workitem != null) && (workitem.size() > 0))
                {
                    pwr.SetComments(request.statusToUpdate);
                    pwr.setWorkItemId(workitem[0].id);
                    pwr.setAction('Removed');
                    
                    // Execute process request
                    Approval.ProcessResult pr = Approval.process(pwr);
                } 
            }
        }
    }
    
    //RecallCaseRequest
    public class RecallCaseRequest{
    
        @InvocableVariable
        public Id caseId;
        
        @InvocableVariable
        public String statusToUpdate;
        
    }
}