powershell Exception calling "Replace" with "3" argument(s): "Value cannot be null

While calling this function, I get the exception calling null error.

Code:

if(Test-Path($Env:TEMP))  {    $hsPrefDataDirectories = Get-ChildItem "$Env:TEMP" | where {$_.Name -match "hsperfdata"} | Format-Table name -hideTableHeaders | Out-String -stream        $hsPrefDataDirectories = [regex]::Replace($hsPrefDataDirectories, "^\s+", '')    $hsPrefDataDirectories = [regex]::Replace($hsPrefDataDirectories, "\s+", ',')    $hsPrefDataDirectories = [regex]::Replace($hsPrefDataDirectories, ",$", '')    $hsPrefDataDirectories = $hsPrefDataDirectories.split(",")    .....    $hsPrefDataDirectories = $hsPrefDataDirectories.split(",")

Error: Exception calling "Replace" with "3" argument(s): "Value cannot be null.Parameter name: input"...You cannot call a method on a null-valued expression.