stats: Add exception handling for legacy materials

bukkit/src/main/java/net/william278/husksync/data/BukkitData.java

@@ -730,7 +730,13 @@ public abstract class BukkitData implements Data {
         private <T extends Keyed> Map<String, Integer> convertStatistics(@NotNull Map<T, Integer> stats) {
             return stats.entrySet().stream().filter(entry -> entry.getKey() != null).collect(
                     TreeMap::new,
-                    (m, e) -> m.put(e.getKey().getKey().toString(), e.getValue()), TreeMap::putAll
+                    (m, e) -> {
+                        try {
+                            m.put(e.getKey().getKey().toString(), e.getValue());
+                        } catch (Throwable t) {
+                            // Ignore; skip elements with invalid keys (e.g., legacy materials)
+                        }
+                    }, TreeMap::putAll
             );
         }