Change !isset to ??= assignment operator: phpstan errors "always exists and is not nullable" case.

@dpi, thank you for your answer, We can discuss.

For PreviewTest.php, lets keep the change in the ??= issue.

Well, this case block the MR, because of the phpstan error. I face this before.

For PathPluginBase, \Drupal\views\Plugin\views\display\DisplayPluginInterface::getPlugin the docs say it can never return null.

So, is not nullable, so,

    if (!isset($access_plugin)) {
      // @todo Do we want to support a default plugin in getPlugin itself?
      $access_plugin = Views::pluginManager('access')->createInstance('none');
    }

is inconsistent: never go in the block. Or is nullable, and this code is okay (...and could be changed to ??=).

@dpi, I think the problem is not ??=. The problem, as phpstan tells us, is not nullable + isset.

I push the work, since done. It can be refused.

An error exists.

Okay @dpi, after having a look your requests about null are clear for me.

About #6, on one side, in core/modules/views_ui/tests/src/FunctionalJavascript/PreviewTest.php,

  /**
   * Asserts that an element has a given class.
   *
   * @param \Behat\Mink\Element\NodeElement $element
   *   The element to test.
   * @param string $class
   *   The class to assert.
   * @param string $message
   *   (optional) A verbose message to output.
   *
   * @internal
   */
  protected function assertClass(NodeElement $element, string $class, string $message = ''): void {
    if (!isset($message)) {
      $message = "Class .$class found.";
    }
    $this->assertStringContainsString($class, $element->getAttribute('class'), $message);

On the other side, in core/modules/system/tests/src/Functional/Pager/PagerTest.php,

  /**
   * Asserts that an element has a given class.
   *
   * @param \Behat\Mink\Element\NodeElement $element
   *   The element to test.
   * @param string $class
   *   The class to assert.
   * @param string $message
   *   (optional) A verbose message to output.
   *
   * @internal
   */
  protected function assertClass(NodeElement $element, string $class, string $message = NULL): void {
    if (!isset($message)) {
      $message = "Class .$class found.";
    }
    $this->assertTrue($element->hasClass($class), $message);
  }

To make the two match change,
, string $message = ''): void {
to
, string $message = NULL): void {

@dpi, for #6 and $access_plugin,

However I can see there a return void;, so lets create an issue just for this, which changes the documentation, and explicitly return NULL;

I believe you have in head,

    // Return now if no options have been loaded.
    if (empty($options) || !isset($options['type'])) {
      return;
    }

in getPlugin in core/modules/views/src/Plugin/views/display/DisplayPluginBase.php. And if I follow #3391355: \Drupal\Core\Config\StorageInterface::read is typehinted as possibly returning bool, but never returns true I think the change needed should be,
* @return \Drupal\views\Plugin\views\ViewsPluginInterface
to
* @return \Drupal\views\Plugin\views\ViewsPluginInterface|null
in,

  /**
   * Get the instance of a plugin, for example style or row.
   *
   * @param string $type
   *   The type of the plugin.
   *
   * @return \Drupal\views\Plugin\views\ViewsPluginInterface
   */
  public function getPlugin($type);

in core/modules/views/src/Plugin/views/display/DisplayPluginInterface.php since,
abstract class DisplayPluginBase extends PluginBase implements DisplayPluginInterface, DependentPluginInterface {

Finally the issue perimeter could be restricted to the nullable problems,

so lets create an issue just for this

using this issue for these problems.

Still some phpstan errors,

 --------------------------------------------------------------------------------------
  Line   core/modules/views/src/Plugin/views/display/DisplayPluginBase.php                               
--------------------------------------------------------------------------------------
         Ignored error pattern #^Variable \$pager in isset\(\) always exists                             
         and is not nullable\.$# in path                                                                 
         /builds/issue/drupal-3424107/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php  
         was not matched in reported errors.                                                             
         Ignored error pattern #^Variable \$style in empty\(\) always exists                             
         and is not falsy\.$# in path                                                                    
         /builds/issue/drupal-3424107/core/modules/views/src/Plugin/views/display/DisplayPluginBase.php  
         was not matched in reported errors.                                                             
------------------------------------------------------------------------------------- 
------------------------------------------------------------------------------------- 
  Line   core/modules/views/src/Plugin/views/display/PathPluginBase.php                               
------------------------------------------------------------------------------------- 
         Ignored error pattern #^Variable \$access_plugin in isset\(\) always                         
         exists and is not nullable\.$# in path                                                       
         /builds/issue/drupal-3424107/core/modules/views/src/Plugin/views/display/PathPluginBase.php  
         was not matched in reported errors.                                                          
------------------------------------------------------------------------------------- 
------------------------------------------------------------------------------------- 
  Line   core/modules/views/src/Plugin/views/style/StylePluginBase.php                               
------------------------------------------------------------------------------------- 
         Ignored error pattern #^Variable \$plugin in empty\(\) always exists                        
         and is not falsy\.$# in path                                                                
         /builds/issue/drupal-3424107/core/modules/views/src/Plugin/views/style/StylePluginBase.php  
         was not matched in reported errors.                                                         
------------------------------------------------------------------------------------- 
------------------------------------------------------------------------------------- 
  Line   core/modules/views_ui/tests/src/FunctionalJavascript/PreviewTest.php                               
------------------------------------------------------------------------------------- 
         Ignored error pattern #^Variable \$message in isset\(\) always exists                              
         and is not nullable\.$# in path                                                                    
         /builds/issue/drupal-3424107/core/modules/views_ui/tests/src/FunctionalJavascript/PreviewTest.php  
         was not matched in reported errors.                                                                
------------------------------------------------------------------------------------- 
 [ERROR] Found 5 errors

- The phpstan errors have no line number. The corresponding items should be removed from core/phpstan-baseline.neon file.
- Following the messages,

in empty\(\) always exists and is not falsy
in isset\(\) always exists and is not nullable

since always exists, make some changes empty($A) to !$A and isset($A) to $A !== NULL. (Not always, to keep the ??= transformations still possible.)

MR mergeable. NR.

Title changed. Less detailed but more clear.