private static void replaySingleEvent(SubstituteLoggingEvent event) {
        if (event == null)
            return;

        SubstituteLogger substLogger = event.getLogger();
        String loggerName = substLogger.getName();
        if (substLogger.isDelegateNull()) {
            throw new IllegalStateException("Delegate logger cannot be null at this state.");
        }

        if (substLogger.isDelegateNOP()) {
            // nothing to do
        } else if (substLogger.isDelegateEventAware()) {
            if(substLogger.isEnabledForLevel(event.getLevel())) {
                substLogger.log(event);
            }
        } else {
            Reporter.warn(loggerName);
        }
    }
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