I've been getting the same thing. I'll note that you accidentally cut off the full error; that last line should say "<Rampant/control.lua:968>".
The simple fix for when it occurs is to reload the most recent save, then find the Rampant map setting "Optimization: Scan chunks outside raised events" and toggle it. When the warp happens, the error shouldn't come up again; you're also free to toggle the setting it back to what it was before. I /think/ there's a 1 in 12 chance that this doesn't fix it. In that case, roll back to a save early enough that you can warp out manually before the auto-warp.
Ultimately, the problem happens when a warp and a 'processPendingUpgrades' call happen on the same tick. Both of the above provided solutions just try to make that not happen. As for how to fix this permanently, I'm not too familiar with either Factorio modding or Lua, but would a try-catch fix this? Not sure if interrupting processPendingUpgrades would do anything bad.
