grade can be null

EDPlayerJournal/CombatZones.cs

@@ -43,7 +43,7 @@ public class CombatZones {
     /// Returns the given combat zone difficulty as an integer, so it can be sorted.
     /// 0 = lowest difficulty, 1 = medium and so forth.
     /// </summary>
-    public static int? DifficultyRank(string difficulty) {
+    public static int? DifficultyRank(string? difficulty) {
         Dictionary<string, int> ranks = new() {
             { DifficultyLow, 0 },
             { DifficultyMedium, 1 },
@@ -51,6 +51,10 @@ public class CombatZones {
             { DifficultyVeryHigh, 3 }
         };
 
+        if (difficulty == null ) {
+            return null;
+        }
+
         if (ranks.TryGetValue(difficulty, out int rank)) {
             return rank;
         }