Powershell - getting cannot index into a null array

foreach($i in $list.items)
    {
        $linkDetails= new-object Microsoft.SharePoint.SPFieldUrlValue($i["ItemLink"])
        write-host $linkDetails.URL
        write-host $linkDetails.Description
        Write-host $i.Url

}

More than half of the items in the library does not have ItemLink field populated. How to get around this issue when there is no value in the field?