buildingplan: fix Unlink all skipping mechanisms when freeing

docs/changelog.txt

@@ -65,6 +65,7 @@ Template for new versions:
 - `aquifer`: make ``--skip-top`` and top-relative ``--levels`` take effect for the ``drain``, ``convert``, and ``add`` actions instead of being silently ignored
 - `autodump`: ``destroy`` no longer leaves the contents of destroyed containers in limbo or crashes when destroying unit-held items
 - `timestream`: deal properly with units who have breathing difficulties
+- `buildingplan`: fix "Unlink all" only unlinking some mechanisms (or crashing) when freeing mechanisms from the building
 
 ## Misc Improvements
 - Added ``Coord2d`` and ``Coord3d`` C++ templates, providing a standard set operations for 2-tuples and 3-tuples of any numeric type

plugins/lua/buildingplan/unlink_mechanisms.lua

@@ -402,8 +402,17 @@ function MechLinkOverlay:ask_unlink_all()
         local free_src = (saved_mode % 2 == 1)
         local free_dst = (saved_mode // 2 > 0)
 
+        -- freeing a mechanism removes it from contained_items and shifts the
+        -- indices of the entries after it, so resolve items before mutating
+        local items = {}
         for _, idx in ipairs(self.links) do
-            local item = self.building.contained_items[idx].item
+            local entry = self.building.contained_items[idx]
+            if entry then
+                table.insert(items, entry.item)
+            end
+        end
+
+        for _, item in ipairs(items) do
             local target = get_mech_target(item)
 
             if target then