LibJS: Convert Promise::create() to NonnullGCPtr

Author: https://github.com/linusg
Commit: a4d85cd522
Pull-request: https://github.com/SerenityOS/serenity/pull/16479
Reviewed-by: https://github.com/davidot ✅

Userland/Libraries/LibJS/Runtime/Completion.cpp

@@ -89,7 +89,7 @@ ThrowCompletionOr<Value> await(VM& vm, Value value)
     auto on_rejected = NativeFunction::create(realm, move(rejected_closure), 1, "");
 
     // 7. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
-    auto* promise = verify_cast<Promise>(promise_object);
+    auto promise = verify_cast<Promise>(promise_object);
     promise->perform_then(on_fulfilled, on_rejected, {});
 
     // FIXME: Since we don't support context suspension, we attempt to "wait" for the promise to resolve