fix crash on invalid saved seed id

docs/changelog.txt

@@ -57,6 +57,7 @@ Template for new versions:
 
 ## Fixes
 - `buildingplan`: make the construction dimensions readout visible again
+- `seedwatch`: fix a crash when reading data saved by very very old versions of the plugin
 
 ## Misc Improvements
 - `sort`: sort by need for training on squad assignment screen

plugins/seedwatch.cpp

@@ -103,6 +103,10 @@ static bool validate_seed_config(color_ostream& out, PersistentDataItem c)
 {
     int seed_id = get_config_val(c, SEED_CONFIG_ID);
     auto plant = binsearch_in_vector(world->raws.plants.all, &df::plant_raw::index, seed_id);
+    if (!plant) {
+        WARN(config, out).print("discarded invalid seed id: %d\n", seed_id);
+        return false;
+    }
     bool valid = (!plant->flags.is_set(df::enums::plant_raw_flags::TREE));
     if (!valid) {
         DEBUG(config, out).print("invalid configuration for %s discarded\n", plant->id.c_str());