Appender.java

ByteArrayOutputStream stream =  new ByteArrayOutputStream();
PrintStream printStream = new PrintStream(stream);
printStream.print("initial text");

LoggerContext context = (LoggerContext) LoggerFactory.getILoggerFactory();

PatternLayoutEncoder encoder = new PatternLayoutEncoder();
encoder.setContext(context);
encoder.setPattern("%d{HH:mm:ss} %-5level %logger{36} - %msg%n");
encoder.start();

OutputStreamAppender<ILoggingEvent> appender= new OutputStreamAppender<>();
appender.setName( "OutputStream Appender" );
appender.setContext(context);
appender.setOutputStream(printStream);
appender.setEncoder(encoder);

appender.start();

Logger log = context.getLogger(this.getClass());
log.addAppender(appender);

log.info( "text from logger");

I tried your example snippet above, but I'm not seeing any exceptions. The status output you've shown above is actually from file:/home/zinger/devel/projects/den/inventory_manager/target/test-classes/logback-test.xml and not from the Java snippet. The warning message right before the NPE is worth investigating:

22:28:18,861 |-WARN in ch.qos.logback.core.OutputStreamAppender[OutputStream Appender] - Encoder has not been set. Cannot invoke its init method.
22:28:18,867 |-ERROR in ch.qos.logback.core.OutputStreamAppender[OutputStream Appender] - Appender [OutputStream Appender] failed to append. java.lang.NullPointerException
at java.lang.NullPointerException