[FIXED JENKINS-18193] NPE thrown when Jenkins failed to initialize.

Not really solving anything here—there is no plausible implementation that could run under these circumstances—
but at least reporting the problem more clearly.

core/src/main/java/jenkins/security/ConfidentialStore.java

@@ -63,7 +63,11 @@ public abstract class ConfidentialStore {
     public static @Nonnull ConfidentialStore get() {
         if (TEST!=null) return TEST.get();
 
-        Lookup lookup = Jenkins.getInstance().lookup;
+        Jenkins j = Jenkins.getInstance();
+        if (j == null) {
+            throw new IllegalStateException("cannot initialize confidential key store until Jenkins has started");
+        }
+        Lookup lookup = j.lookup;
         ConfidentialStore cs = lookup.get(ConfidentialStore.class);
         if (cs==null) {
             try {