alert when tag is not accepted

[NeckbreakerPascal]

I like to give feedback to the user if the typed tag is not accepted.

So I tried to do it with the alertCtrl by ionic but then I got an error:

Cannot read property 'create' of undefined
    at IonTagsInput.webpackJsonp.54.ReporterPage.verifyTag [as verifyMethod] (reporter.ts:574)
    at IonTagsInput.verifyTag (ionic-tags-input.js:117)
    at IonTagsInput.keyAddTag (ionic-tags-input.js:81)
    at Object.eval [as handleEvent] (IonTagsInput.html:20)
    at handleEvent (core.js:13547)
    at callWithDebugContext (core.js:15056)
    at Object.debugHandleEvent [as handleEvent] (core.js:14643)
    at dispatchEvent (core.js:9962)
    at core.js:10587
    at platform-browser.js:2628

It doesn't matter if I try to do it with local Alertcontroller inside the verify Method. Any solution?