fix: only reset timer persistent if Isolate is valid

Author: edusperoni

NativeScript/runtime/Timers.hpp

@@ -36,7 +36,9 @@ class TimerTask {
     }
 
     inline void Unschedule() {
-        callback_.Reset();
+        if (wrapper.IsValid()) {
+            callback_.Reset();
+        }
         args_.reset();
         isolate_ = nullptr;
         queued_ = false;