Fix GDExtension classes derived from abstract GDExtension classes always being registered as abstract

core/object/class_db.cpp

@@ -1661,7 +1661,15 @@ void ClassDB::register_extension_class(ObjectGDExtension *p_extension) {
 	c.name = p_extension->class_name;
 	c.is_virtual = p_extension->is_virtual;
 	if (!p_extension->is_abstract) {
-		c.creation_func = parent->creation_func;
+		// Find the closest ancestor which is either non-abstract or native (or both).
+		ClassInfo *concrete_ancestor = parent;
+		while (concrete_ancestor->creation_func == nullptr &&
+				concrete_ancestor->inherits_ptr != nullptr &&
+				concrete_ancestor->gdextension != nullptr) {
+			concrete_ancestor = concrete_ancestor->inherits_ptr;
+		}
+		ERR_FAIL_NULL_MSG(concrete_ancestor->creation_func, "Extension class " + String(p_extension->class_name) + " cannot extend native abstract class " + String(concrete_ancestor->name));
+		c.creation_func = concrete_ancestor->creation_func;
 	}
 	c.inherits = parent->name;
 	c.class_ptr = parent->class_ptr;