NPE in Bouncycastle when extracting the private key from the signing key returns null

[wilkinsona]

java.lang.NullPointerException: Cannot invoke "org.bouncycastle.openpgp.PGPPrivateKey.getKeyID()" because "<parameter2>" is null
	at org.bouncycastle.openpgp.operator.jcajce.JcaPGPContentSignerBuilder.build(Unknown Source)
	at org.bouncycastle.openpgp.PGPSignatureGenerator.init(Unknown Source)
	at io.spring.github.actions.artifactorydeploy.openpgp.ArmoredAsciiSigner.getSignatureGenerator(ArmoredAsciiSigner.java:193)
	at io.spring.github.actions.artifactorydeploy.openpgp.ArmoredAsciiSigner.sign(ArmoredAsciiSigner.java:186)
	at io.spring.github.actions.artifactorydeploy.openpgp.ArmoredAsciiSigner.sign(ArmoredAsciiSigner.java:175)
	at io.spring.github.actions.artifactorydeploy.openpgp.ArmoredAsciiSigner.sign(ArmoredAsciiSigner.java:161)
	at io.spring.github.actions.artifactorydeploy.openpgp.ArmoredAsciiSigner.sign(ArmoredAsciiSigner.java:138)
	at io.spring.github.actions.artifactorydeploy.openpgp.ArmoredAsciiSignerTests.getWithStringSigningKeyWhenSigningKeyIsKeyReturnsSigner(ArmoredAsciiSignerTests.java:85)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

The root cause is this returning null:

artifactory-deploy-action/src/main/java/io/spring/github/actions/artifactorydeploy/openpgp/ArmoredAsciiSigner.java

Line 113 in b106e28

return signingKey.extractPrivateKey(getDecryptorFactory(passphrase));

The null result is then passed into the constructor of PGPSignatureGenerator:

artifactory-deploy-action/src/main/java/io/spring/github/actions/artifactorydeploy/openpgp/ArmoredAsciiSigner.java

Line 193 in b106e28

signatureGenerator.init(PGPSignature.BINARY_DOCUMENT, this.privateKey);

Doing so triggers the NPE. We should probably fail fast when extractPrivateKey returns null so that the cause of the problem is more clear.