AttributeError: 'NoneType' object has no attribute 'external_id'

[sentry-io]

Sentry Issue: CARE-GK

AttributeError: 'NoneType' object has no attribute 'external_id'
(8 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 476, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 502, in dispatch
    response = handler(request, *args, **kwargs)
  File "rest_framework/mixins.py", line 19, in create
    self.perform_create(serializer)
  File "rest_framework/mixins.py", line 24, in perform_create
    serializer.save()
  File "care/facility/api/serializers/asset.py", line 80, in save
    cache_key = "asset:" + str(self.instance.external_id)