[FIX] Handle None response model in fetch_outputs_with_tree

When the trace server returns a non-200 status for both datatree and
timegraph tree endpoints, response.model is None. Accessing .model on
None caused an AttributeError, failing CI tests.

Author: MatthewKhouzam

tmll/tmll_client.py

@@ -265,6 +265,10 @@ def fetch_outputs_with_tree(self, experiment: Experiment, custom_output_ids: Opt
                     self.logger.warning(f"Output type '{output.type}' is not supported.")
                     continue
 
+            if response.model is None:
+                self.logger.warning(f"Tree of the output '{output.name}' is None.")
+                continue
+
             model = response.model.model
             if model is None:
                 self.logger.warning(f"Tree of the output '{output.name}' is None.")