Fix Graphics API shutdown logic

graphics/opengl/src/main/java/de/staropensource/sosengine/graphics/opengl/OpenGlSubsystem.java

@@ -150,12 +150,19 @@ public final class OpenGlSubsystem implements ApiMainClass {
     }
 
     /**
-     * Shuts the graphics subsystem down.
+     * Called when the engine shuts down.
      *
-     * @since v1-alpha0
+     * @since v1-alpha2
      */
-    public static void shutdownApi() {
+    @EventListener(event = InternalEngineShutdownEvent.class, priority = EventPriority.EXTREMELY_UNIMPORTANT)
-        LoggerInstance logger = instance.logger;
+    private static void shutdownApiFromEvent() {
+        if (instance != null)
+            instance.shutdownApi();
+    }
+
+    /** {@inheritDoc} */
+    @Override
+    public void shutdownApi() {
         logger.verb("Shutting down");
 
         @SuppressWarnings("resource")

graphics/src/main/java/de/staropensource/sosengine/graphics/classes/ApiMainClass.java

@@ -22,8 +22,6 @@ package de.staropensource.sosengine.graphics.classes;
 import de.staropensource.sosengine.base.Engine;
 import de.staropensource.sosengine.base.annotations.EventListener;
 import de.staropensource.sosengine.base.classes.SubsystemMainClass;
-import de.staropensource.sosengine.base.internal.events.InternalEngineShutdownEvent;
-import de.staropensource.sosengine.base.types.EventPriority;
 import de.staropensource.sosengine.graphics.events.GraphicsErrorEvent;
 import org.jetbrains.annotations.NotNull;
 
@@ -41,6 +39,17 @@ public interface ApiMainClass extends SubsystemMainClass {
      */
     void initializeApi();
 
+    /**
+     * Shuts the Graphics API down.
+     * <p>
+     * Called when the engine shuts down.
+     *
+     * @see Engine#shutdown()
+     * @see Engine#shutdown(int)
+     * @since v1-alpha0
+     */
+    void shutdownApi();
+
     /**
      * Returns the name of the Graphics API.
      *
@@ -85,14 +94,4 @@ public interface ApiMainClass extends SubsystemMainClass {
      */
     @EventListener(event = GraphicsErrorEvent.class)
     static void onGraphicsError(String error) {}
-
-    /**
-     * Called when the engine shuts down.
-     *
-     * @see Engine#shutdown()
-     * @see Engine#shutdown(int)
-     * @since v1-alpha0
-     */
-    @EventListener(event = InternalEngineShutdownEvent.class, priority = EventPriority.EXTREMELY_UNIMPORTANT)
-    static void shutdownApi() {}
 }