Deprecated function: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated

Instead, can you please try:

if (!empty($route_name) && preg_match('/^(layout_builder\.([^.]+\.)?)/', $route_name)) {
 ... 
}

@ankithashetty Updated as you have suggested. Thank you.

Please consider this patch.

Adding a patch 8.x-1.x branch

I applied #8 successfully, changes looks fine.

The patches in this issue modify same line as another issue: 3421076.
It mentions that "a module can't rely on the route in hook_css_alter".
I prefer to use that solution instead of this one.

Probably this issue can be closed as duplicate.