Take into account that config read could fail on start()

If there is a problem reading All-Projects when the plugin is started,
the plugin configuration is null and this can cause a
NullPointerException later.

Consider a null config "no configs" when finding relevant entries. In
the REST call, return a 500 with an appropiate message.

Change-Id: Iff503f74ffcff25b160c46486909e4ee367475c9

java/com/googlesource/gerrit/plugins/supermanifest/SuperManifestRefUpdatedListener.java

diff --git a/java/com/googlesource/gerrit/plugins/supermanifest/SuperManifestRefUpdatedListener.java b/java/com/googlesource/gerrit/plugins/supermanifest/SuperManifestRefUpdatedListener.java
index d31f660..d6f471d 100644
--- a/java/com/googlesource/gerrit/plugins/supermanifest/SuperManifestRefUpdatedListener.java
+++ b/java/com/googlesource/gerrit/plugins/supermanifest/SuperManifestRefUpdatedListener.java
@@ -252,6 +252,9 @@
 
   /** for debugging. */
   private String configurationToString() {
+    if (config == null) {
+      return "No config loaded (could not read All-Projects)";
+    }
     StringBuilder b = new StringBuilder();
     b.append("Supermanifest config (").append(config.size()).append(") {\n");
     for (ConfigEntry c : config) {
@@ -331,6 +334,13 @@
         identifiedUser.get().getAccountId().get(),
         configurationToString());
 
+    if (config == null) {
+      error(
+          "Plugin could not read conf from All-Projects (processing %s:%s)",
+          manifestProject, manifestBranch);
+      throw new PreconditionFailedException("Plugin could not read conf from All-Projects");
+    }
+
     List<ConfigEntry> relevantConfigs =
         findRelevantConfigs(resource.getProjectState().getProject().getName(), resource.getRef());
     if (relevantConfigs.isEmpty()) {
@@ -355,6 +365,9 @@
 
   private List<ConfigEntry> findRelevantConfigs(String project, String refName) {
     List<ConfigEntry> relevantConfigs = new ArrayList<>();
+    if (config == null) {
+      return relevantConfigs;
+    }
     for (ConfigEntry c : config) {
       if (!c.srcRepoKey.get().equals(project)) {
         continue;