reactor-core/src/main/java/reactor/core/publisher/MonoProcessor.java
@Nullable
	public O peek() {
		if (!isTerminated()) {
			return null;
		}

		if (value != null) {
			return value;
		}

		if (error != null) {
			RuntimeException re = Exceptions.propagate(error);
			re = Exceptions.addSuppressed(re, new Exception("Mono#peek terminated with an error"));
			throw re;
		}

		return null;
	}
@Nullable
	public O peek() {
		if (!isTerminated()) {
			return null;
		}

		if (value != null) {
			return value;
		}

		if (error != null) {
			RuntimeException re = Exceptions.propagate(error);
			re = Exceptions.addSuppressed(re, new Exception("Mono#peek terminated with an error"));
			throw re;
		}

		return null;
	}
Returns the value that completed this {@link MonoProcessor}. Returns {@code null} if the {@link MonoProcessor} has not been completed. If the
{@link MonoProcessor} is completed with an error a RuntimeException that wraps the error is thrown.

@return the value that completed the {@link MonoProcessor}, or {@code null} if it has not been completed

@throws RuntimeException if the {@link MonoProcessor} was completed with an error