Fix stroking out on missing loadbefore/after

lib/mod_list.dart

@@ -337,6 +337,15 @@ class ModList {
       if (loadAfterMod != null &&
           loadOrder.order.indexOf(mod) <
               loadOrder.order.indexOf(loadAfterMod)) {
+        final loadAfterIndex = loadOrder.order.indexOf(loadAfterMod);
+        // Mod is not loaded, we don't care about it
+        if (loadAfterIndex == -1) {
+          logger.warning(
+            'Missing loadAfter: ${mod.id} requires mod with ID $loadAfter',
+          );
+          continue;
+        }
+
         logger.info(
           'Reordering: ${mod.id} should come after ${loadAfterMod.id}',
         );
@@ -352,6 +361,15 @@ class ModList {
       if (loadBeforeMod != null &&
           loadOrder.order.indexOf(mod) >
               loadOrder.order.indexOf(loadBeforeMod)) {
+        final loadBeforeIndex = loadOrder.order.indexOf(loadBeforeMod);
+        // Mod is not loaded, we don't care about it
+        if (loadBeforeIndex == -1) {
+          logger.warning(
+            'Missing loadBefore: ${mod.id} requires mod with ID $loadBefore',
+          );
+          continue;
+        }
+
         logger.info(
           'Reordering: ${mod.id} should come before ${loadBeforeMod.id}',
         );