initiating scan via Owner API on v5 wallet is causing 'tokio-runtime-worker' panic

[marekyggdrasil]

Describe the bug

During the grinnode.live Winter 2020 Bug Bash Challenge a tester @mojitoo discovered a problem with the wallet. He was able to cause v5 wallet to 'tokio-runtime-worker' panic by initiating scan via Owner API. It was discovered while working on test case Grinnode-live/2020-grin-bug-bash-challenge#37. His detailed info is provided in gist and I managed to reproduce the problem on my own

To Reproduce

Steps to reproduce the behavior:

1. Prepare a v5 wallet and put it in Owner API listener mode.
2. Make an API call to run a scan method.
3. You should see 'tokio-runtime-worker' panic in wallet v5 logs.

Bug will be reproduced if you follow steps from Grinnode-live/2020-grin-bug-bash-challenge#37 test case.

Expected behavior

We would expect the 'tokio-runtime-worker' panic to not occur.

Screenshots

N/A

Desktop (please complete the following information):

Tester environment

Darwin MacBook-Pro-de-Workstation.local 19.5.0 Darwin Kernel Version 19.5.0: Tue May 26 20:41:44 PDT 2020; root:xnu-6153.121.2~2/RELEASE_X86_64 x86_64

Reproduction environment

Linux 4.19.0-12-amd64 #1 SMP Debian 4.19.152-1 (2020-10-18) x86_64 unknown unknown GNU/Linux

Additional context

Tester report and my report on reproducing the bug

[jaspervdm]

Thanks for reporting! We fixed this issue in other places shortly before releasing 5.0 but apparently missed this one. I have a PR up now that fixes it.