Improve slightly BaseWindow

api/src/main/java/com/bartlomiejpluta/base/api/game/gui/window/BaseWindow.java

@@ -12,6 +12,7 @@ import static java.util.Objects.requireNonNull;
 public abstract class BaseWindow extends BaseWidget implements Window {
    protected Context context;
    protected GUI gui;
+   protected WindowManager manager;
    protected Component content;
    protected WindowPosition windowPosition;
 
@@ -63,4 +64,14 @@ public abstract class BaseWindow extends BaseWidget implements Window {
    public void handleKeyEvent(KeyEvent event) {
       content.handleKeyEvent(event);
    }
+
+   @Override
+   public void onOpen(WindowManager manager) {
+      this.manager = manager;
+   }
+
+   @Override
+   public void onClose(WindowManager manager) {
+      this.manager = null;
+   }
 }