Add singleton check before adding webxr interface

modules/webxr/register_types.cpp

@@ -45,8 +45,10 @@ void initialize_webxr_module(ModuleInitializationLevel p_level) {
 	GDREGISTER_ABSTRACT_CLASS(WebXRInterface);
 
 #ifdef WEB_ENABLED
-	webxr.instantiate();
-	XRServer::get_singleton()->add_interface(webxr);
+	if (XRServer::get_singleton()) {
+		webxr.instantiate();
+		XRServer::get_singleton()->add_interface(webxr);
+	}
 #endif
 }