Finally got it to work:

If (!$currentEvent.Fields["result"].Value.Key) {
  $row.EndResult = [System.DBNull]::Value
}
else
{
$row.EndResult = $currentEvent.Fields["result"].Value.Key
}

Thank you aakash9 once again.

The problem is still  "Invalid storage type: DBNull. Please use DBNull instead.""
As per your suggestion, I changed the datatype of  $EndREsult from Int to...

Thank you aakash9!

Now, I'm getting this error:
Exception calling "NewRow" with "0" argument(s): "Invalid storage type: DBNull."

It is referring to :
$events =...

Not working.   Thank you.

The file C:\Program Files\Microsoft SQL Server\130\Shared\Microsoft.SqlServer.XEvent.Linq.dll exists but we still get this error message:

Assembly not found. Loading it from C:\Program Files\Microsoft SQL...