Call me pedantic but while nobody here was outright wrong, nobody provided the correct code for the final solution either.
You need to change this line:
$NullSessionPipes = $NullSessionPipes.replace("browser", "")
to this:
$NullSessionPipes = $NullSessionPipes -ireplace [regex]::Escape("browser"), ""
The strange [regex] text isn’t strictly necessary as long as there are no regular expression characters (ex. *+[](), etc) in your string. But you’re safer with it. This syntax works with variables too:
$NullSessionPipes = $NullSessionPipes -ireplace [regex]::Escape($stringToReplace), $stringToReplaceItWith