Deprecated function: Creation of dynamic property

Problem/Motivation

The issue with the deprecated property in your class occurs because of changes introduced in PHP 8.2. Dynamic properties (properties that are not explicitly declared in the class) are deprecated. In your code, the property $fields is dynamically created inside the constructor without being explicitly defined in the class.

Steps to reproduce

Install swapcard with php >=8.2

Proposed resolution

Declare variable i.e
/**
* Fields for the plugin.
*
* @var object
*/
protected $fields;