Scripting Guys need your help

Question

Hello All,
I have a script which is working absolutely fine in workgroup(Windows logs backup) - windows server 2012 but if i run in domain environment found error and not able to pull security logs. only pulling application and system with error.
Script is here,
Function Server-Eventlogs {            
 Param(
  $Computername ="$ENV:COMPUTERNAME.$ENV:USERDNSDOMAIN",
  [array]$EventLogs = @("application","security","system"),
  $BackupFolder = "C:\Eventvwr\"
 Foreach ( $i in $EventLogs ) {
 If(!( Test-Path $BackupFolder )) { New-Item $BackupFolder -Type Directory }
$eventlog="C:\Eventvwr\$Computername" +"_"+$i+"_"+(Get-Date -Format "yyyyMMdd")+ ".evt"
  ##$eventlog="D:\Eventvwr\$i" + (Get-Date).tostring("yyyyMMdd") + "$Computername" + ".evt"
 (Get-wmiobject win32_nteventlogfile -ComputerName $Computername| 
  Where {$_.logfilename -eq "$i"}).backupeventlog($eventlog)            
 ##Clear-EventLog -LogName $i            
 }# end Foreach            
}#end function  
Server-Eventlogs
Error is here 
You cannot call a method on a null-valued expression.
At C:\Users\Administrator\Desktop\backupscriptfinal.ps1:18 char:2
+  (Get-wmiobject win32_nteventlogfile -ComputerName $Computername|
+  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull
Will appreciate for your help.....
Cheers!

Answer 1

Here is a sample of what you will have to do.  It still will fail under many different scenarios.  Designing the error handling for complext tasks is not trivial.
Function Backup-Eventlogs {
Param(
$Computername=$ENV:COMPUTERNAME,
$EventLogs=@('application','security','system'),
$BackupFolder='c:\ELBackups'
Begin{
$datestring=Get-Date -Format 'yyyyMMdd'
Process{
foreach($computer in $Computername){
$remotefolder='\\{0}\{1}' -f $computer, $BackupFolder.Replace(':','$')
If(!(Test-Path $remoteFolder)){
New-Item $remoteFolder -Type Directory
foreach($logname in $EventLogs){
Try{
$EventLog=Get-wmiobject win32_nteventlogfile -ComputerName $computer -Filter "LogFileName='$logname'" -ea Stop
                    $filename='{0}\{1}_{2}_{3}.evt' -f $BackupFolder,$computer,$logname,$datestring
Write-Host "Backing up $logname to $filename on $computer" -ForegroundColor green
$EventLog.BackupEventlog($filename)
Catch{
Write-Host "$computer $logname $_" -ForegroundColor red
Backup-Eventlogs
¯\_(ツ)_/¯