!8841 Fix context none type bug

From: @zjun3021
Reviewed-by: @chujinjin,@kisnwang
Signed-off-by: @chujinjin

mindspore/nn/cell.py

@@ -244,7 +244,7 @@ class Cell(Cell_):
         raise AttributeError("'{}' object has no attribute '{}'.".format(type(self).__name__, name))
 
     def __del__(self):
-        if context.get_context("mode") == context.PYNATIVE_MODE:
+        if context is not None and context.get_context("mode") == context.PYNATIVE_MODE:
             _pynative_exec.clear(str(id(self)))
         if hasattr(self, "_create_time"):
             _executor.del_net_res(str(self._create_time))