fix #1013

Author: mmatera

mathics/core/expression.py

@@ -87,6 +87,8 @@ def __str__(self) -> str:
 
 def from_python(arg):
     number_type = get_type(arg)
+    if arg is None:
+        return Symbol('Null')
     if isinstance(arg, int) or number_type == 'z':
         return Integer(arg)
     elif isinstance(arg, float) or number_type == 'f':