NPE in privacy code

[shemnon]

A user on rocketchat has observed the following NPE, starting in 1.5.4 - https://chat.hyperledger.org/channel/besu?msg=JLGTq9xLA5uExXpLe

java.lang.NullPointerException
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.privacy.PrivateStateRootResolver.resolveLastStateRoot(PrivateStateRootResolver.java:40)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.precompiles.privacy.PrivacyPrecompiledContract.compute(PrivacyPrecompiledContract.java:157)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.MainnetMessageCallProcessor.executePrecompile(MainnetMessageCallProcessor.java:133)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.MainnetMessageCallProcessor.start(MainnetMessageCallProcessor.java:61)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.AbstractMessageProcessor.process(AbstractMessageProcessor.java:163)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.MainnetTransactionProcessor.process(MainnetTransactionProcessor.java:426)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.MainnetTransactionProcessor.processTransaction(MainnetTransactionProcessor.java:339)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.MainnetTransactionProcessor.processTransaction(MainnetTransactionProcessor.java:49)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.TransactionProcessor.processTransaction(TransactionProcessor.java:253)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.mainnet.TransactionProcessor.processTransaction(TransactionProcessor.java:183)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.transaction.TransactionSimulator.process(TransactionSimulator.java:131)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.transaction.TransactionSimulator.process(TransactionSimulator.java:84)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.api.jsonrpc.internal.methods.EthEstimateGas.response(EthEstimateGas.java:71)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.api.jsonrpc.JsonRpcHttpService.process(JsonRpcHttpService.java:602)
validator1-bootnode_1  | 	at org.hyperledger.besu.ethereum.api.jsonrpc.JsonRpcHttpService.lambda$handleJsonSingleRequest$10(JsonRpcHttpService.java:473)
validator1-bootnode_1  | 	at io.vertx.core.impl.ContextImpl.lambda$executeBlocking$2(ContextImpl.java:313)
validator1-bootnode_1  | 	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
validator1-bootnode_1  | 	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
validator1-bootnode_1  | 	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
validator1-bootnode_1  | 	at java.base/java.lang.Thread.run(Unknown Source)