Fixed pickle loading of BufferDTypes

Author: Trenza1ore

stable_baselines3/common/buffers.py

@@ -41,7 +41,7 @@ class BufferDTypes:
     observations: InitVar[Union[DTypeLike, Mapping[str, DTypeLike]]]
     actions: InitVar[DTypeLike]
 
-    dict_obs: MappingProxyType[str, np.dtype] = field(init=False)
+    dict_obs: MappingProxyType[str, np.dtype] = field(default=MappingProxyType({}), init=False)
     obs: Optional[np.dtype] = field(default=None, init=False)
     act: Optional[np.dtype] = field(default=None, init=False)
 
@@ -61,7 +61,7 @@ def __getstate__(self):
     def __setstate__(self, state: Mapping[str, Any]):
         state = dict(state)
         if state.get("dict_obs"):
-            state["dict_obs"] = MappingProxyType(state["dict_obs"])
+            state["dict_obs"] = MappingProxyType(state["dict_obs"].copy())
         self.__dict__.update(state)
 
     @classmethod