Fix "'NoneType' object has no attribute 'cast'" error when model is freed (#117)

Author: mczk77

rwkv/rwkv_cpp_shared_library.py

@@ -90,6 +90,8 @@ def __init__(self, shared_library_path: str):
         self.library.rwkv_get_system_info_string.argtypes = []
         self.library.rwkv_get_system_info_string.restype = ctypes.c_char_p
 
+        self.nullptr = ctypes.cast(0, ctypes.c_void_p)
+
     def rwkv_init_from_file(self, model_file_path: str, thread_count: int) -> RWKVContext:
         """
         Loads the model from a file and prepares it for inference.
@@ -232,7 +234,7 @@ def rwkv_free(self, ctx: RWKVContext) -> None:
 
         self.library.rwkv_free(ctx.ptr)
 
-        ctx.ptr = ctypes.cast(0, ctypes.c_void_p)
+        ctx.ptr = self.nullptr
 
     def rwkv_quantize_model_file(self, model_file_path_in: str, model_file_path_out: str, format_name: str) -> None:
         """