panic: send on closed channel in block import notifier

[noot]

Describe the bug

Occasionally when running the node I've seen this panic, causing the node to crash:

panic: send on closed channel

goroutine 56677 [running]:
github.com/ChainSafe/gossamer/dot/state.(*BlockState).notifyImported.func1(0xc005da4120, 0xc005f02780)
	/home/ubuntu/gossamer/dot/state/block_notify.go:107 +0x3f
created by github.com/ChainSafe/gossamer/dot/state.(*BlockState).notifyImported
	/home/ubuntu/gossamer/dot/state/block_notify.go:105 +0x1ae

need to make sure there are no writes to channels that are potentially closed

To Reproduce

Steps to reproduce the behaviour:

This happens quite infrequently, so not sure how to reproduce it other than running the node a bunch of times

[github-actions]

🎉 This issue has been resolved in version 0.6.0 🎉

The release is available on GitHub release

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