Event with m.relates_to: null causes 500 error (AttributeError: 'NoneType' object has no attribute 'get')

[matrixbot]

This issue has been migrated from #5404.

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Description

After update to homeserver.py version 0.99.5.2 - in logs I get errors:

      File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/python/failure.py", line 512, in throwExceptionIntoGenerator
        return g.throw(self.type, self.value, self.tb)
      File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/synapse/util/metrics.py", line 63, in measured_func
        r = yield func(self, *args, **kwargs)
      File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
        result = g.send(result)
      File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/synapse/handlers/message.py", line 608, in create_new_client_event
        if relation.get("rel_type") == RelationTypes.ANNOTATION:
    AttributeError: 'NoneType' object has no attribute 'get'

error.log

Version information

deb packet: matrix-synapse-py3 0.99.5.2+stretch1 amd64
Debian 9.8
LXC container.