[BUG] salt.thorium.reg.clear clears a level of dict too much

Description

I try to fire a single action in response to many events (one from each minion).
So I use the following example code:

patch_grp2:
  reg.set:
    - add: host
    - match: custom/system/up
  check.contains:
    - value: mysystem1
    - value: mysystem2

shell_test:
  local.cmd:
    - tgt: '*'
    - func: cmd.run
    - arg:
      - echo 'thorium success' > /tmp/thorium.txt
    - require:
      - check: patch_grp2
  reg.clear:
    - name: patch_grp2

And on mysystem1/2 I do:

salt-call event.send custom/system/up "{host: $(hostname -s)}"

It works as expected: /tmp/thorium.txt is created after both minions sent the event.
But since the action is triggered multiple times (e.g. if I rm /tmp/thorium.txt on a minion, file is instantly recreated), I do the reg.clear in my action.
If seems this would solve my problem, but there seems to be a bug:

The clear triggers and I get the following error in Log

2020-07-02 14:59:11,525 [salt.state                                                         :322 ][ERROR   ][1412] An exception occurred in this state: Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/salt/state.py", line 1981, in call
    **cdata['kwargs'])
  File "/usr/local/lib/python3.6/dist-packages/salt/loader.py", line 1977, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/salt/thorium/check.py", line 270, in contains
    if value in __reg__[name]['val']:
KeyError: 'val'

After that the Thorium register doesn't work at all until master restart.
I guess you want to clear the contents of __reg__[name]['val'] but actually you do clear __reg__[name] ?

Versions Report

Salt Version:
           Salt: 3000.2
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: unknown
       dateutil: 2.6.1
      docker-py: Not Installed
          gitdb: 2.0.3
      gitpython: 2.1.8
         Jinja2: 2.11.2
        libgit2: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.6.9 (default, Apr 18 2020, 01:56:04)
   python-gnupg: 0.4.1
         PyYAML: 5.3.1
          PyZMQ: 19.0.0
          smmap: 2.0.3
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.3.2
 
System Versions:
           dist: Ubuntu 18.04 bionic
         locale: UTF-8
        machine: x86_64
        release: 4.15.0-96-generic
         system: Linux
        version: Ubuntu 18.04 bionic

Start with the salt.thorium.reg.clear entry point and salt/thorium/check.py at the contains function and reported KeyError. Trace how clearing patch_grp2 changes the register, then add or update focused coverage so clearing does not break subsequent event handling and the register continues working without a master restart.