Fix an issue with the is_big_annoying_library function, see #520

jedi/inference/helpers.py

@@ -265,5 +265,10 @@ def values_from_qualified_names(inference_state, *names):
 
 def is_big_annoying_library(context):
     string_names = context.get_root_context().string_names
-    # Pandas is huge, so just ignore it, because  stuff makes everything slow.
+    if string_names is None:
+        return False
+
+    # Especially pandas and tensorflow are huge complicated Python libraries
+    # that get even slower than they already are when Jedi tries to undrstand
+    # dynamic features like decorators, ifs and other stuff.
     return string_names[0] in ('pandas', 'numpy', 'tensorflow', 'matplotlib')