Fix crashes when scanning Git projects

Addressed a NullPointerException that could occur in infrequent cases
when retrieving the file tree in a Git project. This exception is now
properly handled.

Change-Id: I7cc35d3951c3ddc369bd12322213bb40e57cfcda
Signed-off-by: Patrizio <patrizio.gelosi@amarulasolutions.com>

src/main/java/com/googlesource/gerrit/plugins/chatgpt/mode/stateful/client/api/git/GitRepoFiles.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/chatgpt/mode/stateful/client/api/git/GitRepoFiles.java b/src/main/java/com/googlesource/gerrit/plugins/chatgpt/mode/stateful/client/api/git/GitRepoFiles.java
index 25b8b6b..a8bd8a8 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/chatgpt/mode/stateful/client/api/git/GitRepoFiles.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/chatgpt/mode/stateful/client/api/git/GitRepoFiles.java
@@ -127,6 +127,10 @@
         try (RevWalk revWalk = new RevWalk(repository)) {
             return revWalk.parseCommit(lastCommitId).getTree();
         }
+        catch (NullPointerException e) {
+            log.warn("Error retrieving Master Rev Tree for ID `{}`", lastCommitId, e);
+            throw new IOException(e);
+        }
     }
 
     private String readFileContent(ObjectReader reader, RevTree tree, String path) throws IOException {