NULL value not work for DPG

[tadelesh]

Because in DPG, we use deep copy to make the model object immutable, then customized JSON serialization for NULL will fail here.

[msyyc]

from copy import deepcopy
class _Null:
    pass

NULL = _Null()
x = {"x": NULL}
y = deepcopy(x)
if y["x"] is NULL:
    print("yes")
else:
    print("no")

good catch!