Describe the bug
"Object reference not set to an instance of an object" error showed during consensus node running.

To Reproduce
Steps to reproduce the behavior:

1. Start a seed node.
2. Start a single consensus node.

Expected behavior
this error should not show and consensus node works well.

Screenshots

Platform:

• OS: [e.g. Ubuntu 18.04 x64]
• Version [e.g. neo-cli 3.0]
  neo-cli: master (Limit incoming connections based on param. Support connecting to trusted IPs. #470)
  neo: master (Remove the case of GetData in ProtocolHandlerMailbox#ShallDrop #1201)

(Optional) Additional context
Massive transaction frequently sent to an normal node.

consensus_SingleNode.zip