Config bundler NPE when missing server settings

[electrum]

An NPE occurs if the settings file doesn't have a <server> entry with a matching <id>. The result of settings.getServer(...) needs to be checked for null with an appropriate user error thrown.

Exception in thread "main" java.lang.NullPointerException
at io.airlift.airship.configbundler.Maven.makeRemoteRepository(Maven.java:102)
at io.airlift.airship.configbundler.Maven.<init>(Maven.java:90)
at io.airlift.airship.configbundler.SnapshotCommand.call(SnapshotCommand.java:60)
at io.airlift.airship.configbundler.SnapshotCommand.call(SnapshotCommand.java:21)
at io.airlift.airship.configbundler.ConfigBundler.main(ConfigBundler.java:31)

Additionally, the constructor for that class has the repository info parameters as @Nullable when they should not be. The validateRepositoryMetadata method should validate them as not null.