coding standards and best practices

Problem/Motivation

While reviewing the module for best practices and standards. I have found issues.

FILE: /var/www/html/dcontri/responsive_class_field/src/Normalizer/ResponsiveClassItemNormalizer.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 23 | ERROR | unserialize() is insecure unless allowed classes are limited. Use a safe format like JSON or use the allowed_classes option.
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FILE: /var/www/html/dcontri/responsive_class_field/src/Plugin/Field/FieldType/ResponsiveClassItem.php
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FOUND 2 ERRORS AND 1 WARNING AFFECTING 3 LINES
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 106 | WARNING | t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
 306 | ERROR   | unserialize() is insecure unless allowed classes are limited. Use a safe format like JSON or use the allowed_classes option.
 333 | ERROR   | unserialize() is insecure unless allowed classes are limited. Use a safe format like JSON or use the allowed_classes option.
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FILE: /var/www/html/dcontri/responsive_class_field/src/Plugin/Field/FieldWidget/ResponsiveClassWidget.php
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FOUND 2 ERRORS AFFECTING 2 LINES
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  95 | ERROR | [x] Use null coalesce operator instead of ternary operator.
 139 | ERROR | [x] Use null coalesce operator instead of ternary operator.
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FILE: /var/www/html/dcontri/responsive_class_field/src/Plugin/DataType/ResponsiveClassClassesData.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 29 | ERROR | [x] Use null coalesce operator instead of ternary operator.
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FILE: /var/www/html/dcontri/responsive_class_field/src/Plugin/DataType/ResponsiveClassData.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 18 | ERROR | Missing member variable doc comment
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