fix: handle null public directory gracefully in InstallCommand

The getPublicDir() method can return null, but the code was directly
concatenating it with a string, which could cause unexpected behavior.

Now the command properly checks for null and returns a failure with
a clear error message instead of proceeding with an invalid path.

tests/Symfony/Command/InstallCommandTest.php

@@ -7,6 +7,7 @@ namespace Flasher\Tests\Symfony\Command;
 use Flasher\Tests\Symfony\Fixtures\FlasherKernel;
 use Mockery\Adapter\Phpunit\MockeryTestCase;
 use Symfony\Bundle\FrameworkBundle\Console\Application;
+use Symfony\Component\Console\Command\Command;
 use Symfony\Component\Console\Tester\CommandTester;
 use Symfony\Component\HttpKernel\Kernel;
 
@@ -77,4 +78,69 @@ final class InstallCommandTest extends MockeryTestCase
         $command = $application->find('flasher:install');
         $this->commandTester = new CommandTester($command);
     }
+
+    public function testExecuteFailsGracefullyWhenPublicDirIsNull(): void
+    {
+        // Create a kernel that returns a non-existent project directory
+        // This will cause getPublicDir() to return null
+        $kernel = new class() extends Kernel {
+            public function __construct()
+            {
+                parent::__construct('test', true);
+            }
+
+            public function registerBundles(): iterable
+            {
+                return [
+                    new \Symfony\Bundle\FrameworkBundle\FrameworkBundle(),
+                    new \Flasher\Symfony\FlasherSymfonyBundle(),
+                ];
+            }
+
+            public function registerContainerConfiguration(\Symfony\Component\Config\Loader\LoaderInterface $loader): void
+            {
+                $loader->load(function (\Symfony\Component\DependencyInjection\ContainerBuilder $container): void {
+                    $container->register('kernel', static::class)->setPublic(true);
+                    $container->loadFromExtension('framework', [
+                        'secret' => 'test',
+                        'test' => true,
+                        'session' => [
+                            'handler_id' => null,
+                            'storage_factory_id' => 'session.storage.factory.mock_file',
+                        ],
+                    ]);
+                });
+            }
+
+            public function getProjectDir(): string
+            {
+                // Return a non-existent directory without public/ subfolder
+                return '/non-existent-directory-'.uniqid();
+            }
+
+            public function getCacheDir(): string
+            {
+                return sys_get_temp_dir().'/cache'.spl_object_hash($this);
+            }
+
+            public function getLogDir(): string
+            {
+                return sys_get_temp_dir().'/logs'.spl_object_hash($this);
+            }
+        };
+
+        $kernel->boot();
+
+        $application = new Application($kernel);
+        $application->setCatchExceptions(false);
+
+        $commandTester = new CommandTester($application->find('flasher:install'));
+        $exitCode = $commandTester->execute([]);
+
+        $output = $commandTester->getDisplay();
+
+        // Should fail gracefully with an error message instead of crashing
+        $this->assertSame(Command::FAILURE, $exitCode);
+        $this->assertStringContainsString('Could not determine the public directory', $output);
+    }
 }