message: Update core.ts
  controller.spawn() throws UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'context' of undefined . Fix for Bug#1945
diff: @@ -1077,7 +1077,7 @@ export class Botkit {
  }
  let worker: BotWorker = null;
  - const adapter = custom_adapter || (config.context && config.context.adapter) ? config.context.adapter : this.adapter;
  + const adapter = custom_adapter || (config && config.context && config.context.adapter) ? config.context.adapter : this.adapter;
  if (adapter.botkit_worker) {
  const CustomBotWorker = adapter.botkit_worker;
  