sharepoint Powershell

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Here is my code for copy file from on site to other site But

here is errror

You cannot call a method on a null-valued expression.
At line:5 char:40
+ $dFile = $dList.RootFolder.File.Add <<<< ($RootItem.Name, $sBytes, $true)
+ CategoryInfo : InvalidOperation: (Add:String) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull

You cannot call a method on a null-valued expression.
At line:22 char:18
+ $dFile.Update <<<< ()
+ CategoryInfo : InvalidOperation: (Update:String) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull

$SourceWebURL = "http://portal-dev:45106/personal/test"

#SoureLibraryTitle is the title of the Source Library where documents reside
$SourceLibraryTitle = "Shared Documents"

$DestinationWebURL = "http://portal-dev/IT/test/backup"

#DestinationLibraryTitle is the title of the destionation library in which you want to store your files
$DestinationLibraryTitle = "mysitebackup"

write-host $DestinationLibraryTitle

$sWeb = Get-SPWeb $SourceWebURL

$sList = $sWeb.Lists | ? {$_.Title -eq $SourceLibraryTitle}

$dWeb = Get-SPWeb $DestinationWebURL

#Retrieve the source list using the parameter provided
$dList = $dWeb.Lists | ? {$_.title.ToLower() -like $DestinationLibraryTitle}

#Retrieve all folders in the source list.  This used to maintain the folder structure in source and destination libraries.  This excludes the root folder
$AllFolders = $sList.Folders

#Retrieve the root folder in the source list.
$RootFolder = $sList.RootFolder

#Return all files that exist directly in the root of the library
$RootItems = $RootFolder.files

foreach($RootItem in $RootItems)
{
   
    $sBytes = $RootItem.OpenBinary()
    Write-host $RootItem
 
 $dFile = $dList.RootFolder.File.Add($RootItem.Name, $sBytes, $true)
  
   
    $AllFields = $RootItem.Item.Fields |  ? {!($_.sealed)}
    
    foreach($Field in $AllFields)
    {
   
        if($RootItem.Properties[$Field.Title])
        {
   
            if(!($dFile.Properties[$Field.title]))
            {
               
                $dFile.AddProperty($Field.Title, $RootItem.Properties[$Field.Title])
            }
            else
            {
                #If the property already exists, set the value on the destination item to the same value as it was in the source item.
                $dFile.Properties[$Field.Title] = $RootItem.Properties[$Field.Title]
            }
           
        }
    }
   
    $dFile.Update()
}

You cannot call a method on a null-valued expression.
At line:5 char:40
+     $dFile = $dList.RootFolder.File.Add <<<< ($RootItem.Name, $sBytes, $true)
    + CategoryInfo          : InvalidOperation: (Add:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

You cannot call a method on a null-valued expression.
At line:22 char:18
+     $dFile.Update <<<< ()
    + CategoryInfo          : InvalidOperation: (Update:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull