NullPointerException in HtmlLayout

AsyncLogger error handling event seq=233, value='[ERROR calling class org.apache.logging.log4j.core.async.RingBufferLogEvent.toString(): java.lang.NullPointerException]':
org.apache.logging.log4j.core.appender.AppenderLoggingException: An exception occurred processing Appender Mail
	at org.apache.logging.log4j.core.appender.DefaultErrorHandler.error(DefaultErrorHandler.java:75)
	at org.apache.logging.log4j.core.config.AppenderControl.handleAppenderError(AppenderControl.java:165)
	at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:158)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:129)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:120)
	at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:84)
	at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:448)
	at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:433)
	at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:417)
	at org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:79)
	at org.apache.logging.log4j.core.async.AsyncLogger.actualAsyncLog(AsyncLogger.java:337)
	at org.apache.logging.log4j.core.async.RingBufferLogEvent.execute(RingBufferLogEvent.java:156)
	at org.apache.logging.log4j.core.async.RingBufferLogEventHandler.onEvent(RingBufferLogEventHandler.java:45)
	at org.apache.logging.log4j.core.async.RingBufferLogEventHandler.onEvent(RingBufferLogEventHandler.java:29)
	at com.lmax.disruptor.BatchEventProcessor.run(BatchEventProcessor.java:129)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.logging.log4j.LoggingException: Error occurred while sending email
	at org.apache.logging.log4j.core.net.SmtpManager.sendEvents(SmtpManager.java:175)
	at org.apache.logging.log4j.core.appender.SmtpAppender.append(SmtpAppender.java:181)
	at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:156)
	... 15 more
Caused by: java.lang.NullPointerException
	at org.apache.logging.log4j.core.layout.HtmlLayout.toSerializable(HtmlLayout.java:170)
	at org.apache.logging.log4j.core.layout.HtmlLayout.toSerializable(HtmlLayout.java:50)
	at org.apache.logging.log4j.core.layout.AbstractStringLayout.toByteArray(AbstractStringLayout.java:301)
	at org.apache.logging.log4j.core.net.SmtpManager.writeBuffer(SmtpManager.java:204)
	at org.apache.logging.log4j.core.net.SmtpManager.writeContent(SmtpManager.java:190)
	at org.apache.logging.log4j.core.net.SmtpManager.formatContentToBytes(SmtpManager.java:182)
	at org.apache.logging.log4j.core.net.SmtpManager.sendEvents(SmtpManager.java:163)
	... 17 more