I've just updated and now I get this error when loading my savegame.

The mod Hostile Trees (0.1.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event hostile-trees::on_tick (ID 0)
hostile-trees/control.lua:38: attempt to index local 'config' (a nil value)
stack traceback:
hostile-trees/control.lua:38: in function <hostile-trees/control.lua:26>

Did you start the game with an earlier mod version? Versions before 0.1.3 were broken in this regard. For what it's worth 0.1.5 should work fine, sorry about your save :(

Disabling the mod, loading and then saving my game, then enabling the mod and loading the game again fixed it.

Better would be to use the migration possibility for mods.
https://lua-api.factorio.com/latest/auxiliary/migrations.html

This I'm avoiding right now for the sake of simplicity. As long as it's a 0.x and anything can change, backwards compatibility is a waste of time.