when:
        createRunner().forwardStdError(new StringWriter()).forwardStdOutput(null)

        then:
        def t = thrown(IllegalArgumentException)
        t.message == 'standardOutput argument cannot be null'
    }

    def "throws exception if standard error is null"() {
        when:
        createRunner().forwardStdOutput(new StringWriter()).forwardStdError(null)

        then:

     * If you require separation of the streams, you can use {@link #forwardStdOutput(Writer)} and {@link #forwardStdError(Writer)} directly.
     * <p>
     * Calling this method will negate the effect of previously calling {@link #forwardStdOutput(Writer)} and/or {@link #forwardStdError(Writer)}.
     *
     * @return this
     * @since 2.9
     * @see #forwardStdOutput(Writer)
     * @see #forwardStdError(Writer)
     */

        delegate.withEnvironment(environmentVariables)
        return this
    }

    @Override
    SmokeTestGradleRunner forwardStdOutput(Writer writer) {
        delegate.forwardStdOutput(writer)
        return this
    }

    @Override
    SmokeTestGradleRunner forwardStdError(Writer writer) {
        delegate.forwardStdError(writer)
        return this

    @Override
    public GradleRunner withEnvironment(Map<String, String> environment) {
        this.environment = environment;
        return this;
    }

    @Override
    public GradleRunner forwardStdOutput(Writer writer) {
        if (forwardingSystemStreams) {
            forwardingSystemStreams = false;
            this.standardError = null;
        }
        validateArgumentNotNull(writer, "standardOutput");