Deprecated: preg_split(): Passing null in argument_validator

Problem/Motivation

Deprecated: preg_split(): Passing null to parameter #2 ($subject) of type string is deprecated in /app/web/core/modules/views/src/Plugin/views/argument_validator/Entity.php on line 183

Steps to reproduce

1. While using the WYSIWYG, click on media browser button.
2. Click "Library" at the top of the modal.
3. See the warning at the top of the modal.

Proposed resolution

Don't run preg_split() on $argument variable if null.

Remaining tasks

1. Check my fix.
2. Correct or guide me to a better solution.
3. Test.