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 });