fix: safe check when computing chart thumbnail (#23432)

superset/tasks/thumbnails.py

@@ -48,6 +48,9 @@ def cache_chart_thumbnail(
         logger.warning("No cache set, refusing to compute")
         return None
     chart = cast(Slice, Slice.get(chart_id))
+    if not chart:
+        logger.warning("No chart found, skip computing chart thumbnail")
+        return None
     url = get_url_path("Superset.slice", slice_id=chart.id)
     logger.info("Caching chart: %s", url)
     _, username = get_executor(