private static void emitSubstitutionWarning() {
        Reporter.warn("The following set of substitute loggers may have been accessed");
        Reporter.warn("during the initialization phase. Logging calls during this");
        Reporter.warn("phase were not honored. However, subsequent logging calls to these");
        Reporter.warn("loggers will work as normally expected.");
        Reporter.warn("See also " + SUBSTITUTE_LOGGER_URL);
    }
The value of PROVIDER.getMDCAdapter() can be null while PROVIDER has not yet initialized.

However, SLF4JServiceProvider implementations are expected to initialize their internal
MDCAdapter field in their constructor or on field declaration.
slf4j-api/src/main/java/org/slf4j/LoggerFactory.java