Add isFlushed() to Kernel interface

Also add explicit return type

src/Core/BaseKernel.php

@@ -188,7 +188,7 @@ abstract class BaseKernel implements Kernel
             $this->getIncludeTests(),
             $flush
         );
-        
+
         // Flush config
         if ($flush) {
             $config = $this->getConfigLoader()->getManifest();
@@ -265,7 +265,7 @@ abstract class BaseKernel implements Kernel
 
     abstract public function boot($flush = false);
 
-    abstract public function isFlushed();
+    abstract public function isFlushed(): ?bool;
 
     /**
      * Check if there's a legacy _ss_environment.php file

src/Core/CoreKernel.php

@@ -15,11 +15,8 @@ class CoreKernel extends BaseKernel
 
     /**
      * Indicates whether the Kernel has been flushed on boot
-     * Uninitialised before boot
-     *
-     * @var bool
      */
-    private $flush;
+    private ?bool $flush = null;
 
     /**
      * @param false $flush
@@ -217,12 +214,7 @@ class CoreKernel extends BaseKernel
         return null;
     }
 
-    /**
-     * Returns whether the Kernel has been flushed on boot
-     *
-     * @return bool|null null if the kernel hasn't been booted yet
-     */
-    public function isFlushed()
+    public function isFlushed(): ?bool
     {
         return $this->flush;
     }

src/Core/DatabaselessKernel.php

@@ -16,11 +16,9 @@ class DatabaselessKernel extends BaseKernel
 {
     /**
      * Indicates whether the Kernel has been flushed on boot
-     * Uninitialised before boot
-     *
-     * @var bool
+     * Null before boot
      */
-    private $flush;
+    private ?bool $flush = null;
 
     /**
      * Allows disabling of the configured error handling.
@@ -53,10 +51,7 @@ class DatabaselessKernel extends BaseKernel
         $this->setBooted(true);
     }
 
-    /**
-     * @return bool
-     */
-    public function isFlushed()
+    public function isFlushed(): ?bool
     {
         return $this->flush;
     }

src/Core/Kernel.php

@@ -132,4 +132,11 @@ interface Kernel
      * @return $this
      */
     public function setEnvironment($environment);
+
+    /**
+     * Returns whether the Kernel has been flushed on boot
+     *
+     * @return bool|null null if the kernel hasn't been booted yet
+     */
+    public function isFlushed(): ?bool;
 }