fix groupstage crash

PadelClub/Data/GroupStage.swift

@@ -458,12 +458,15 @@ final class GroupStage: BaseGroupStage, SideStorable {
     var scoreCache: [Int: TeamGroupStageScore] = [:]
     
     func computedScore(forTeam team: TeamRegistration, step: Int = 0) -> TeamGroupStageScore? {
-        if let cachedScore = scoreCache[team.groupStagePositionAtStep(step)!] {
+        guard let groupStagePositionAtStep = team.groupStagePositionAtStep(step) else {
+            return nil
+        }
+        if let cachedScore = scoreCache[groupStagePositionAtStep] {
             return cachedScore
         } else {
-            let score = _score(forGroupStagePosition: team.groupStagePositionAtStep(step)!)
+            let score = _score(forGroupStagePosition: groupStagePositionAtStep)
             if let score = score {
-                scoreCache[team.groupStagePositionAtStep(step)!] = score
+                scoreCache[groupStagePositionAtStep] = score
             }
             return score
         }