public static <I extends Request, O extends Response, U extends PreClient<I, O>>
    O executeWithFallback(U execution,
                          PreClientRequestContext ctx, I req,
                          BiFunction<ClientRequestContext, Throwable, O> errorResponseFactory) {
        final ClientRequestContextExtension ctxExt = ctx.as(ClientRequestContextExtension.class);
        if (ctxExt != null) {
            ctxExt.runContextCustomizer();
        }
        try {
            return execution.execute(ctx, req);
        } catch (Exception e) {
            fail(ctx, e);
            return errorResponseFactory.apply(ctx, e);
        }
    }
An undefined {@link URI} to create {@link WebClient} without specifying {@link URI}.
core/src/main/java/com/linecorp/armeria/internal/client/ClientUtil.java