This has already been asked and answered and is working except for the errorCase.SuppliedName returns Null after my error message. How do I write a Test Class for this. I'm a newbie and this has helped me tremendously.

trigger PreventCloseServiceRequestTrigger on Case (after update) {

// Find all service request that are being closed
Set<Id> newlyClosedCaseIds = new Set<Id>();
for (Id caseId : Trigger.newMap.keySet()) {
    if (Trigger.newMap.get(caseId).IsClosed && 
        !Trigger.oldMap.get(caseId).IsClosed) {
        newlyClosedCaseIds.add(caseId);
    }
}

// For every newly closed service request that has at least one 
// not closed task add an error to the Trigger.newMap entry.
// Adding the error to the Trigger.new entry will prevent the save
// and add the error message to the page messages that get displayed on
// the page.
for (AggregateResult aggResult : [
        Select Count(Id), WhatId
        From Task
        Where WhatId In :newlyClosedCaseIds
              And IsClosed = false
        Group by WhatId
        Having Count(Id) > 0
]) {
    Id caseId = (Id) aggResult.get('WhatId');
    Case errorCase = Trigger.newMap.get(caseId);

    // add my error message to the coordinator goes down here
    errorCase.addError('Please complete all OPEN TASK before trying to CLOSE this SERVICE REQUEST: ' +
                       errorCase.SuppliedName); 
}

}

This has already been asked and answered and is working except for the errorCase.SuppliedName returns Null after my error message. How do I write a Test Class for this. I'm a newbie and this has helped me tremendously.

trigger PreventCloseServiceRequestTrigger on Case (after update) {

// Find all service request that are being closed
Set<Id> newlyClosedCaseIds = new Set<Id>();
for (Id caseId : Trigger.newMap.keySet()) {
    if (Trigger.newMap.get(caseId).IsClosed && 
        !Trigger.oldMap.get(caseId).IsClosed) {
        newlyClosedCaseIds.add(caseId);
    }
}

// For every newly closed service request that has at least one 
// not closed task add an error to the Trigger.newMap entry.
// Adding the error to the Trigger.new entry will prevent the save
// and add the error message to the page messages that get displayed on
// the page.
for (AggregateResult aggResult : [
        Select Count(Id), WhatId
        From Task
        Where WhatId In :newlyClosedCaseIds
              And IsClosed = false
        Group by WhatId
        Having Count(Id) > 0
]) {
    Id caseId = (Id) aggResult.get('WhatId');
    Case errorCase = Trigger.newMap.get(caseId);

    // add my error message to the coordinator goes down here
    errorCase.addError('Please complete all OPEN TASK before trying to CLOSE this SERVICE REQUEST: ' +
                       errorCase.SuppliedName); 
    }
 }