Guard against erroneous disposal (fixes #28)

During plugin updates, Hyper calls the `clearCache` method. This method
appears to execute `onUnload` without the context of `onApp`. Adding a
guard clause as a quick-fix, as I do not fully understand what is taking
place currently.

Author: dcalhoun

modules/dispose.js

@@ -2,12 +2,17 @@
 const { showWindows } = require('./windows')
 
 module.exports = (app, callbacks) => {
-  const { handleActivate, handleBlur, cfgUnsubscribe } = callbacks
+  const { cfgUnsubscribe, handleActivate, handleBlur } = callbacks
+
+  if (!cfgUnsubscribe || !handleActivate || !handleBlur) {
+    return
+  }
 
   showWindows(app)
   app.dock.show()
-  // TODO: Un-register shortcut when supported
+  // TODO: Unregister shortcut when supported
   // unregisterShortcut()
+
   cfgUnsubscribe()
   app.removeListener('activate', handleActivate)
   app.removeListener('browser-window-blur', handleBlur)