fix: Ensure Isolate is alive before using it (#63)

[darind]

Related to #63.

Steps to reproduce:

var worker = new Worker("./tests/shared/Workers/EvalWorker.js"); worker.onmessage = (msg) => { worker.terminate(); NSNotificationCenter.defaultCenter.postNotificationNameObject("MyNotification", null); }; const workerScript = `  var NotificationObserver = /** @class */ (function (_super) {  __extends(NotificationObserver, _super);  function NotificationObserver() {  return _super !== null && _super.apply(this, arguments) || this;  }  NotificationObserver.initWithCallback = function (onReceiveCallback) {  var observer = _super.new.call(this);  observer._onReceiveCallback = onReceiveCallback;  return observer;  };  NotificationObserver.prototype.onReceive = function (notification) {  this._onReceiveCallback(notification);  };  NotificationObserver.ObjCExposedMethods = {  onReceive: { returns: interop.types.void, params: [NSNotification] },  };  return NotificationObserver;  }(NSObject));   const observer = NotificationObserver.initWithCallback(notification => {  });   NSNotificationCenter.defaultCenter.addObserverSelectorNameObject(observer, "onReceive", "MyNotification", null);   postMessage(self === global);`; worker.postMessage({ eval: workerScript });