Powershell syntax...

I have a CSV file - two columns: Accountname, Groupname

I have a loop that creates the Groupname
I have a sub loop that checks the Accountname validity and adds the Accountname to the Groupname

My sub loop works, it correctly adds the right Accountname to the right Groupname. But I get this error "You cannot call a method on a null-valued expression." even though I don't believe the value is Null.

I believe my method runs successfully but is badly written

foreach($group in $UserList)
{     
    $GroupName = $group.GroupName
    if($web.SiteGroups[$GroupName] -ne $null)
    {
        write-host "$GroupName already exists"
    }
     else
    {
        write-host "$GroupName does not exist, creating new $GroupName group"
        $web.SiteGroups.Add($GroupName, $siteownergroup, $null, $description)
        $spgroup = $web.SiteGroups["$GroupName"]
        $spgroup.Update()      
        
        foreach($Row in $UserList)
        {
            #### Get the Group name
            $UserGroup = $web.Groups[$Row.GroupName]
            $user = $web.site.RootWeb.EnsureUser($Row.AccountName)
            if ($user -ne $Null)
             {
               $UserGroup.AddUser($user)
             }        
         }
    }
 }

The ERROR message is

You cannot call a method on a null-valued expression.
At C:\Scripts\groups-externalcsv.ps1:49 char:14
+ $UserGroup.AddUser($user)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull


It errors each time at $UserGroup.AddUser($user)