Infinite loop causes Null Reference Exception of ironpython

The following code leads to Null Reference Exception of iron python. In class WarnOnDel, function del is executed multiple times, leading to Null Reference Exception.

test.py

import threading
import warnings

class WarnOnDel(object):
    def __del__(self):
        warnings.warn("wrong", UserWarning)

def do_work():
    while True:
        w = WarnOnDel()

for i in range(10):
    t = threading.Thread(target=do_work)
    t.setDaemon(1)
    t.start()

Environmental:

IronPython 3.4.0a1 (3.4.0.0001)
[.NETFramework,Version=v4.6 on Mono 6.12.0.122 (tarball Mon Feb 22 17:28:32 UTC 2021) (64-bit)] on linux

Error report:

(base) xxm@xxm-System-Product-Name:~$ ipy test.py
test.py:1: UserWarning: oh no something went wrong
  import threading

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
  at IronPython.Runtime.WeakRefTracker.Finalize () [0x00000] in <5cd2739cd27844619789a1aa1ffdb0b0>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
  at IronPython.Runtime.WeakRefTracker.Finalize () [0x00000] in <5cd2739cd27844619789a1aa1ffdb0b0>:0 

Start with the test.py reproducer and the IronPython.Runtime.WeakRefTracker.Finalize stack-trace entry. Investigate why repeated threaded object finalization reaches a null reference, then verify that the reproducer completes without the exception and add regression coverage if the repository has a suitable test location.