Integrate KoliBri web-components in Vaadin - vaadin

Question

I am trying to integrate KoliBri web-components (https://github.com/public-ui/kolibri) in a Vaadin project. I followed the documentation for web components integration (https://vaadin.com/docs/latest/create-ui/web-components) but I was not successful.
I want to integrate a KoliBri-button (kol-button) and therefor created getter and setter methods for the required properties of the button. When loading the website, the kol-button-component is loaded successfully from the .js file.
enter image description here
But the kol-button element in the DOM is empty and won´t show up:
enter image description here
Here is my KolButton.java:
package com.example.application.views.helloworld;
import com.vaadin.flow.component.Component;
import com.vaadin.flow.component.Synchronize;
import com.vaadin.flow.component.Tag;
import com.vaadin.flow.component.dependency.JsModule;
import com.vaadin.flow.component.dependency.NpmPackage;
#Tag("kol-button")
#NpmPackage(value = "#public-ui/components", version = "1.1.10")
#JsModule("#public-ui/components/dist/components/kol-button")
public class KolButton extends Component {
public boolean getLabel() {
return getElement().getProperty("_label", false);
}
public void setLabel(String label) {
getElement().setProperty("_label", label);
}
public void setVariant(String variant) {
getElement().setProperty("_variant", variant);
}
public boolean getVariant() {
return getElement().getProperty("_variant", false);
}
}
And the view.java:
package com.example.application.views.helloworld;
import com.example.application.views.MainLayout;
import com.vaadin.flow.component.orderedlayout.HorizontalLayout;
import com.vaadin.flow.router.PageTitle;
import com.vaadin.flow.router.Route;
import com.vaadin.flow.router.RouteAlias;
#PageTitle("Hello World")
#Route(value = "hello", layout = MainLayout.class)
#RouteAlias(value = "", layout = MainLayout.class)
public class HelloWorldView extends HorizontalLayout {
public HelloWorldView() {
var kolButton = new KolButton();
kolButton.setLabel("TestText");
kolButton.setVariant("danger");
setVerticalComponentAlignment(Alignment.END, kolButton);
add(kolButton);
}
}
Do you have any idea to solve this? Thanks in advance