Relevant-Code

            Ipv4MatchArbitraryBitMaskBuilder ipv4MatchBuilder = new Ipv4MatchArbitraryBitMaskBuilder();
            makeHashingMatch(flowRuleData.hashingData.get(), ipv4MatchBuilder);
            // vvvvvvvvvvvv If this section is removed, some fields are null, but the errors go away vvvvvvvvvvvvv
            if (flowRuleData.matchedSrcIp.isPresent()) {
                Ipv4Prefix ipPrefix = flowRuleData.matchedSrcIp.get();
                Ipv4Address ipAddr  = new Ipv4Address(ipPrefix.getValue().split("/")[0]);
                ipv4MatchBuilder.setIpv4SourceAddressNoMask(ipAddr);
                if (ipv4MatchBuilder.getIpv4SourceArbitraryBitmask() == null) {
                    // Match the whole address
                    ipv4MatchBuilder.setIpv4SourceArbitraryBitmask(new DottedQuad("255.255.255.255"));
                }
            }
            if (flowRuleData.matchedDestIp.isPresent()) {
                Ipv4Prefix ipPrefix = flowRuleData.matchedDestIp.get();
                Ipv4Address ipAddr  = new Ipv4Address(ipPrefix.getValue().split("/")[0]);
                ipv4MatchBuilder.setIpv4DestinationAddressNoMask(ipAddr);
                if (ipv4MatchBuilder.getIpv4DestinationArbitraryBitmask() == null) {
                    // Match the whole address
                    ipv4MatchBuilder.setIpv4DestinationArbitraryBitmask(new DottedQuad("255.255.255.255"));
                }
            }
            // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            matchBuilder = new MatchBuilder().setEthernetMatch(ethernetMatchBuilder.build())
                                             .setLayer3Match(ipv4MatchBuilder.build());

...
Somewhere else
...
// This method works as expected and does not appear to cause any bugs
public static void makeHashingMatch(HashingRuleData hashingData, Ipv4MatchArbitraryBitMaskBuilder ipv4MatchBuilder)
    {
        int i = hashingData.i;
        if (hashingData.hashingMode.equals(HashingMode.SOURCE_ONLY)) {
            int significantBitsMask = (int) (Math.pow(2, hashingData.numberOfBits) - 1);
            ipv4MatchBuilder.setIpv4SourceAddressNoMask(         new Ipv4Address("0.0.0." + String.valueOf(i)))
                            .setIpv4SourceArbitraryBitmask(      new DottedQuad( "0.0.0." + String.valueOf(significantBitsMask)));
        } else if (hashingData.hashingMode.equals(HashingMode.DESTINATION_ONLY)) {
            int significantBitsMask = (int) (Math.pow(2, hashingData.numberOfBits) - 1);
            ipv4MatchBuilder.setIpv4DestinationAddressNoMask(         new Ipv4Address("0.0.0." + String.valueOf(i)))
                            .setIpv4DestinationArbitraryBitmask(      new DottedQuad( "0.0.0." + String.valueOf(significantBitsMask)));
        } else if (hashingData.hashingMode.equals(HashingMode.SOURCE_AND_DESTINATION)) {
            int indexRange = (int) Math.pow(2, hashingData.numberOfBits/2);
            int significantBitsMask = (int) (indexRange - 1);
            int srcIndex = i%indexRange;
            int destIndex = (i - srcIndex)/indexRange;
            ipv4MatchBuilder.setIpv4SourceAddressNoMask(         new Ipv4Address("0.0.0." + String.valueOf(srcIndex)))
                            .setIpv4SourceArbitraryBitmask(      new DottedQuad( "0.0.0." + String.valueOf(significantBitsMask)))
                            .setIpv4DestinationAddressNoMask(         new Ipv4Address("0.0.0." + String.valueOf(destIndex)))
                            .setIpv4DestinationArbitraryBitmask(      new DottedQuad( "0.0.0." + String.valueOf(significantBitsMask)));
        } else {
            throw new RuntimeException("unexpected value for HashingMode");
        }
    }