Prometheus is an open-source project and it should possible to investigate the reported NullPointerException by looking into the code [1]. This seems like a configuration required to include SSL for the RMI client used by the JMX exporter.

[1]. https://github.com/prometheus/jmx_exporter/blob/master/collector/src/main/java/io/prometheus/jmx/JmxScraper.java