增加非空判断

lib/data_host/native/data_host.dart

@@ -20,7 +20,7 @@ class VidDataHost extends VidDataHostBase {
   VidUsProbe get probe => _data!.probe;
 
   @override
-  int get frameCount => _data!.imageCount;
+  int get frameCount => _data == null ? -1 : _data!.imageCount;
 
   @override
   Future<VidUsImageData?> getData() async => _data;