fix bug: check before using the variables (jupyterlab#656)

* fix bug: check before using the variables

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

packages/jupyter-ai/jupyter_ai/completions/handlers/llm_mixin.py

@@ -30,6 +30,9 @@ def get_llm_chain(self):
         lm_provider = self.config_manager.lm_provider
         lm_provider_params = self.config_manager.lm_provider_params
 
+        if not lm_provider or not lm_provider_params:
+            return None
+
         curr_lm_id = (
             f'{self.llm.id}:{lm_provider_params["model_id"]}' if self.llm else None
         )
@@ -39,9 +42,6 @@ def get_llm_chain(self):
             else None
         )
 
-        if not lm_provider or not lm_provider_params:
-            return None
-
         if curr_lm_id != next_lm_id:
             self.log.info(
                 f"Switching {self.handler_kind} language model from {curr_lm_id} to {next_lm_id}."