Null GamerScore causes crash and UI freeze on startup

[sharkusmanch]

CountAchievements() crashes when processing games without a GamerScore value (Steam, GOG, etc.). The cast at line 281:

double score = (double)item["GamerScore"];

throws InvalidCastException for null values. The exception is caught by the outer try-catch, but this happens for every non-Xbox/RetroAchievements game in the library — thousands of times per startup.

02-02 20:00:32.495|ERROR|SuccessStoryFullscreenHelper#SuccessStoryFullscreenHelper:Failed to process file: C:\Users\theme\AppData\Roaming\Playnite\ExtensionsData\cebe6d32-8c46-4459-b993-5a5189d60788\SuccessStory\f5f3fa30-2bf6-4ce9-bf16-4f8fc8f39d11.json
   at SuccessStoryFullscreenHelper.SuccessStoryFullscreenHelper.CountAchievements() in C:\Downloads\Plugins\SuccessStoryFullscreenHelper\SuccessStoryFullscreenHelper.cs:line 305
02-02 20:00:32.495|ERROR|SuccessStoryFullscreenHelper#SuccessStoryFullscreenHelper:Failed to process file: C:\Users\theme\AppData\Roaming\Playnite\ExtensionsData\cebe6d32-8c46-4459-b993-5a5189d60788\SuccessStory\f684a3fb-2c31-4527-a39e-39f8ab45a33c.json
   at SuccessStoryFullscreenHelper.SuccessStoryFullscreenHelper.CountAchievements() in C:\Downloads\Plugins\SuccessStoryFullscreenHelper\SuccessStoryFullscreenHelper.cs:line 305
02-02 20:00:32.575|INFO |SuccessStoryFullscreenHelper#SuccessStoryFullscreenHelper:SuccessStory stats loaded from 3301 files. Bronze: 1363, Silver: 584, Gold: 367, Platinum: 95, Total: 2409

Additionally, CountAchievements() runs synchronously in OnApplicationStarted, blocking the UI thread during all file I/O and JSON parsing.