The other day, I encountered an isse with IDA pro 7.5: my struct and enum list left panel became empty, with blank elements. As you can see belo, It was unusable:

I have no idea how this happended exactly. IN the end I had to contact the support, and here is their solution, a single python commnd:

import ida_netnode
ida_netnode.netnode("$ dirtree/structs").kill()

You can then save and reopen the idb, the list should have been updated and be fine. You’ll lose the folders though, but it’s not that long to do so it’s ok.
This command also works with enums instead of structs, and maybe other pannels like this.