Prevent exception when adding a character from Imperator with a taken…

… ID (#1549)

ImperatorToCK3/CK3/Characters/CharacterCollection.cs

@@ -94,7 +94,7 @@ Configuration config
 			config
 		);
 		character.CK3Character = newCharacter;
-		Add(newCharacter);
+		AddOrReplace(newCharacter);
 	}
 
 	public override void Remove(string key) {

ImperatorToCK3/CK3/Titles/RulerTerm.cs

@@ -10,7 +10,7 @@
 namespace ImperatorToCK3.CK3.Titles;
 
 public class RulerTerm {
-	public string CharacterId { get; } = "0";
+	public string? CharacterId { get; }
 	public Date StartDate { get; }
 	public string? Government { get; }
 	public Imperator.Countries.RulerTerm.PreImperatorRulerInfo? PreImperatorRuler { get; }
@@ -55,6 +55,11 @@ Configuration config
 				provinceMapper,
 				config
 			);
+			if (characters.ContainsKey(character.Id)) {
+				Logger.Warn($"Cannot add pre-Imperator ruler {character.Id} " +
+				            $"- a character with this ID already exists!");
+				return;
+			}
 			characters.Add(character);
 			CharacterId = character.Id;
 		}

ImperatorToCK3/CK3/Titles/Title.cs

@@ -283,6 +283,9 @@ void FillHolderAndGovernmentHistory() {
 				);
 
 				var characterId = rulerTerm.CharacterId;
+				if (characterId is null) {
+					continue;
+				}
 				var gov = rulerTerm.Government;
 
 				var termStartDate = new Date(rulerTerm.StartDate);