Fix NPE race condition in AssistantChat.updateLoading (#304)

* Initial plan

* Fix NullPointerException in AssistantChat.updateLoading due to race condition

Co-authored-by: jGauravGupta <15934072+jGauravGupta@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jGauravGupta <15934072+jGauravGupta@users.noreply.github.com>

Author: Copilot

src/main/java/io/github/jeddict/ai/components/AssistantChat.java

@@ -261,7 +261,9 @@ public void updateLoading(final String progress) {
 
         SwingUtilities.invokeLater(() -> {
             LOG.finest(() -> "updpating task loader with handle %s and progress %s".formatted(handle, progress));
-            handle.setDisplayName(progress);
+            if (handle != null) {
+                handle.setDisplayName(progress);
+            }
         });
     }