Fix freezing when fetching IBSN and no results are found

Fixes part of #9979

CHANGELOG.md

@@ -102,6 +102,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 - We fixed an issue where an Automatic Keyword Group could not be deleted in the UI. [#9778](https://github.com/JabRef/jabref/issues/9778)
 - We fixed an issue where the citation key pattern `[edtrN_M]` returned the wrong editor. [#9946](https://github.com/JabRef/jabref/pull/9946)
 - We fixed an issue where empty grey containers would remain in the groups panel, if displaying of group item count is turned off [#9972](https://github.com/JabRef/jabref/issues/9972)
+- We fixed an issue where fetching an ISBN could lead to application freezing when the fetcher did not return any results [#9979](https://github.com/JabRef/jabref/issues/9979)
 
 ### Removed
 

src/main/java/org/jabref/logic/importer/fetcher/isbntobibtex/IsbnFetcher.java

@@ -2,6 +2,7 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Optional;
 import java.util.regex.Pattern;
@@ -69,8 +70,10 @@ public Optional<BibEntry> performSearchById(String identifier) throws FetcherExc
                 LOGGER.debug("Try using the alternate ISBN fetchers to find an entry.");
             }
         } finally {
-            while (bibEntry.isEmpty() && retryIsbnFetcher.iterator().hasNext()) {
-                AbstractIsbnFetcher fetcher = retryIsbnFetcher.iterator().next();
+            // do not move the iterator in the loop as this would always return a new one and thus create and endless loop
+            Iterator<AbstractIsbnFetcher> iterator = retryIsbnFetcher.iterator();
+            while (bibEntry.isEmpty() && iterator.hasNext()) {
+                AbstractIsbnFetcher fetcher = iterator.next();
                 LOGGER.debug("No entry found for ISBN=" + identifier + "; trying " + fetcher.getName() + " next.");
                 bibEntry = fetcher.performSearchById(identifier);
             }