Fixes 8.1 issue: class_exists with null value will error

[scrummitch]

If the UserPanel class member variable $this->filterModel is NULL, php 8.1 will complain and throw a deprecation error.

I am adding an is_null() check before the class_exists call to fix this

PHP Deprecated Warning – yii\base\ErrorException
class_exists(): Passing null to parameter #1 ($class) of type string is deprecated

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	#473

[bizley]

Thank you.