Fix cover generation error

internal/manager/task_generate_screenshot.go

@@ -21,17 +21,20 @@ func (t *GenerateCoverTask) GetDescription() string {
 }
 
 func (t *GenerateCoverTask) Start(ctx context.Context) {
-	if !t.required(ctx) {
-		return
-	}
-
 	scenePath := t.Scene.Path
 
+	var required bool
 	if err := t.txnManager.WithReadTxn(ctx, func(ctx context.Context) error {
+		required = t.required(ctx)
+		
 		return t.Scene.LoadPrimaryFile(ctx, t.txnManager.File)
 	}); err != nil {
 		logger.Error(err)
 	}
+	
+	if !required {
+		return
+	}
 
 	videoFile := t.Scene.Files.Primary()
 	if videoFile == nil {
@@ -89,6 +92,7 @@ func (t *GenerateCoverTask) Start(ctx context.Context) {
 }
 
 // required returns true if the sprite needs to be generated
+// assumes in a transaction
 func (t *GenerateCoverTask) required(ctx context.Context) bool {
 	if t.Scene.Path == "" {
 		return false