fix index of range (#3279)

### What problem does this PR solve?

#3273

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

graphrag/mind_map_extractor.py

@@ -39,7 +39,6 @@ class MindMapResult:
 
 
 class MindMapExtractor:
-
     _llm: CompletionLLM
     _input_text_key: str
     _mind_map_prompt: str
@@ -93,7 +92,7 @@ class MindMapExtractor:
             max_workers = int(os.environ.get('MINDMAP_EXTRACTOR_MAX_WORKERS', 12))
             exe = ThreadPoolExecutor(max_workers=max_workers)
             threads = []
-            token_count = max(self._llm.max_length * 0.8, self._llm.max_length-512)
+            token_count = max(self._llm.max_length * 0.8, self._llm.max_length - 512)
             texts = []
             res = []
             cnt = 0
@@ -163,14 +162,14 @@ class MindMapExtractor:
             elif isinstance(value, list):
                 new_value = {}
                 for i in range(len(value)):
-                    if isinstance(value[i], list):
+                    if isinstance(value[i], list) and i > 0:
                         new_value[value[i - 1]] = value[i][0]
                 data[key] = new_value
             else:
                 continue
         return data
 
-    def _todict(self, layer:collections.OrderedDict):
+    def _todict(self, layer: collections.OrderedDict):
         to_ret = layer
         if isinstance(layer, collections.OrderedDict):
             to_ret = dict(layer)