Fix for #120.

The NullPointerException occurred because there has been an entry in the ACL of an entity which referred to a non-existing realm (aka user source, aka authentication provider). This case is now treated as "user does not exist".

CHANGELOG.md

@@ -20,6 +20,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* #120 - Editing entities that were created with a no longer existing user
+  leads to a server error.
 * #31 - Queries with keywords in the path (e.g. `... STORED AT 0in.txt`)
 * #116 - Queries `FIND [ANY VERSION OF] *` and `FIND [ANY VERSION OF] ENTITY`.
 

src/main/java/org/caosdb/server/accessControl/UserSources.java

@@ -79,8 +79,18 @@ public class UserSources extends HashMap<String, UserSource> {
 
   private static UserSources instance = new UserSources();
 
+  /**
+   * Check whether a user exists.
+   *
+   * @param principal - principal of the user.
+   * @return true iff the user identified by the given {@link Principal} exists.
+   */
   public static boolean isUserExisting(final Principal principal) {
-    return instance.get(principal.getRealm()).isUserExisting(principal.getUsername());
+    UserSource userSource = instance.get(principal.getRealm());
+    if (userSource != null) {
+      return userSource.isUserExisting(principal.getUsername());
+    }
+    return false;
   }
 
   private UserSources() {