修改

app/Http/Controllers/admin/ActivityReward.php

@@ -56,7 +56,7 @@ class ActivityReward extends Controller
                 'content' => ['nullable', 'string'],
                 'params' => ['nullable', 'array'],
             ]);
-            $params['params'] = json_encode($params['params']);
+            $params['params'] = empty($params['params']) ? null : json_encode($params['params']);
             ActivityRewardService::submit($params);
             DB::commit();
         } catch (ValidationException $e) {