Fix groupby-resample KeyError (pandas-dev#50840) by adding None check.

Author: czroth

pandas/core/resample.py

@@ -1202,7 +1202,7 @@ def _gotitem(self, key, ndim, subset=None):
 
         # Try to select from a DataFrame, falling back to a Series
         try:
-            if isinstance(key, list) and self.key not in key:
+            if isinstance(key, list) and self.key not in key and self.key is not None:
                 key.append(self.key)
             groupby = self._groupby[key]
         except IndexError: