Debugger shows `<unknown>:Nothing (?)` while debugging frequently

While debugging recently I've noticed that the code panel goes completely blank. Is this normal?

If you'd like a reproduction case clone this branch, run `make`, and then run `dapp debug` and select `VaultFactoryTest::test_deploy_vault()`. After stepping through the trace for a little bit you will notice the code panel go blank:

https://github.com/rari-Capital/vaults/tree/v1

Clone the linked v1 branch, run make, then start dapp debug and select VaultFactoryTest::test_deploy_vault(). Step through the trace until the code panel becomes blank and capture the surrounding debugger state; done means identifying why it displays :Nothing (?) and keeping the code panel usable during the trace.

Written by the indexing model from the issue text.