Fix destroy method to release ownership of child reference in Group and Window classes

Author: kojix2

src/uing/group.cr

@@ -12,6 +12,7 @@ module UIng
     end
 
     def destroy
+      @child_ref.try &.release_ownership
       @child_ref = nil
       super
     end

src/uing/window.cr

@@ -41,6 +41,8 @@ module UIng
       @on_content_size_changed_box = nil
       @on_closing_box = nil
       @on_focus_changed_box = nil
+      @child_ref.try &.release_ownership
+      @child_ref = nil
       super
     end