Fix crash in resoure duplicate

(cherry picked from commit 9450717571c1b1cd43be5b188e55442dcd412c51)

core/resource.cpp

@@ -224,8 +224,8 @@ Ref<Resource> Resource::duplicate(bool p_subresources) const {
 	List<PropertyInfo> plist;
 	get_property_list(&plist);
 
-	Resource *r = (Resource *)ClassDB::instance(get_class());
-	ERR_FAIL_COND_V(!r, Ref<Resource>());
+	Ref<Resource> r = (Resource *)ClassDB::instance(get_class());
+	ERR_FAIL_COND_V(r.is_null(), Ref<Resource>());
 
 	for (List<PropertyInfo>::Element *E = plist.front(); E; E = E->next()) {
 
@@ -247,7 +247,7 @@ Ref<Resource> Resource::duplicate(bool p_subresources) const {
 		}
 	}
 
-	return Ref<Resource>(r);
+	return r;
 }
 
 void Resource::_set_path(const String &p_path) {