Issue #555

Synthetic properties generated by Kotlin Android Extensions plugin needs a view for Fragment/Activity to be set before hand.

In your case, for Fragment, you need to use view.btn_K in onViewCreated

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    super.onCreateView(inflater, container, savedInstanceState)
    val view = inflater.inflate(R.layout.fragment_card_selector, container, false)
    view.btn_K.setOnClickListener{} // access with `view`
    return view
}

Or better, you should only access synthetic properties in onViewCreated

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    super.onCreateView(inflater, container, savedInstanceState)
    return inflater.inflate(R.layout.fragment_card_selector, container, false)
}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    btn_K.setOnClickListener{} // access without `view`
}

Please notice that savedInstanceState parameter should be nullable Bundle?, and also check Importing synthetic properties

It is convenient to import all widget properties for a specific layout in one go:

import kotlinx.android.synthetic.main.<layout>.*

Thus if the layout filename is activity_main.xml, we’d import kotlinx.android.synthetic.main.activity_main.*.

If we want to call the synthetic properties on View, we should also import kotlinx.android.synthetic.main.activity_main.view.*.

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