ThrowableStackTraceRenderer throws NullPointerException when Throwable.getCause() returns different instances

## Description

Our exception overrides getCause() and returns a new Throwable instance on each invocation. While this implementation is unconventional, Log4j 2.24.3 logs the exception successfully, whereas 2.25.3 throws an internal NullPointerException from ThrowableStackTraceRenderer.

## Configuration

**Version:** 2.25.3, 2.26.0, 2.26.1

**Operating system:** Windows 11 Enterprise

**JDK:** Java 25

## Logs

```
2026-09-01T13:25:19.053233100Z main ERROR An exception occurred processing Appender CONSOLE
java.lang.NullPointerException: Cannot read field "stackLength" because "metadata" is null
at org.apache.logging.log4j.core.pattern.ThrowableStackTraceRenderer.renderStackTraceElements(ThrowableStackTraceRenderer.java:158)
at org.apache.logging.log4j.core.pattern.ThrowableStackTraceRenderer.renderThrowable(ThrowableStackTraceRenderer.java:100)
at org.apache.logging.log4j.core.pattern.ThrowableStackTraceRenderer.renderCause(ThrowableStackTraceRenderer.java:136)
at org.apache.logging.log4j.core.pattern.ThrowableStackTraceRenderer.renderThrowable(ThrowableStackTraceRenderer.java:103)
at org.apache.logging.log4j.core.pattern.ThrowableStackTraceRenderer.renderThrowable(ThrowableStackTraceRenderer.java:77)
at org.apache.logging.log4j.core.pattern.ThrowableStackTraceRenderer.renderThrowable(ThrowableStackTraceRenderer.java:56)
at org.apache.logging.log4j.core.pattern.ThrowablePatternConverter.format(ThrowablePatternConverter.java:130)
at org.apache.logging.log4j.core.layout.PatternLayout$NoFormatPatternSerializer.toSerializable(PatternLayout.java:354)
at org.apache.logging.log4j.core.layout.PatternLayout.toText(PatternLayout.java:251)
at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:237)
at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:57)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.directEncodeEvent(AbstractOutputStreamAppender.java:227)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.tryAppend(AbstractOutputStreamAppender.java:220)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:211)
at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:160)
at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:133)
at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:124)
at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:88)
at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:711)
at org.apache.logging.log4j.core.config.LoggerConfig.processLogEvent(LoggerConfig.java:669)
at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:645)
at org.apache.logging.log4j.core.config.LoggerConfig.log(LoggerConfig.java:589)
at org.apache.logging.log4j.core.config.AwaitCompletionReliabilityStrategy.log(AwaitCompletionReliabilityStrategy.java:92)
at org.apache.logging.log4j.core.Logger.log(Logger.java:187)
at org.apache.logging.log4j.spi.AbstractLogger.tryLogMessage(AbstractLogger.java:2970)
at org.apache.logging.log4j.spi.AbstractLogger.logMessageTrackRecursion(AbstractLogger.java:2922)
at org.apache.logging.log4j.spi.AbstractLogger.logMessageSafely(AbstractLogger.java:2904)
at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:2648)
at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:2587)
at org.apache.logging.log4j.spi.AbstractLogger.error(AbstractLogger.java:824)
at Log4jBugTest.test(Log4jBugTest.java:26)

```

## Reproduction

```
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.junit.jupiter.api.Test;

class Log4jBugTest
{
private static final Logger LOGGER = LogManager.getLogger(Log4jBugTest.class);

@Test
void test()
{
try
{
throw new Exception("Exception")
{
@Override
public synchronized Throwable getCause()
{
return new Throwable("Throwable");
}
};
}
catch (final Exception e)
{
LOGGER.error("Error", e);
}
}
}
```
And my log4j2 config:
```

```

Start with ThrowableStackTraceRenderer.java at the failing renderStackTraceElements path and reproduce the issue using the supplied Log4jBugTest.java example with the shown %ex configuration. Trace how the changing getCause() result leads to missing metadata; done means logging the exception no longer throws an internal NullPointerException and the regression is covered by a test.

Written by the indexing model from the issue text.