PdfAdapter to display a pdf into a list view with some gesture support like zoom

PdfAdapter.kt

import android.graphics.Bitmap
import android.graphics.pdf.PdfRenderer
import androidx.recyclerview.widget.RecyclerView
import android.util.Log
import android.view.ViewGroup
import com.davemorrissey.labs.subscaleview.ImageSource
import com.davemorrissey.labs.subscaleview.SubsamplingScaleImageView

/**
 * Created by timothynibeaudeau on 25/11/2018.
 *
 * Work with com.davemorrissey.labs:subsampling-scale-image-view:3.10.0
 * https://github.com/davemorrissey/subsampling-scale-image-view
 */
class PdfAdapter(private val pdfRenderer: PdfRenderer) : RecyclerView.Adapter<PdfAdapter.ViewHolder>() {
    private val TAG = PdfAdapter::class.java.simpleName

    private val count = pdfRenderer.pageCount

    private val bitmapGenerated = HashMap<Int, Bitmap>(pdfRenderer.pageCount)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = SubsamplingScaleImageView(parent.context)
        view.layoutParams = ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT)
        return ViewHolder(view)
    }

    override fun getItemCount(): Int = count


    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.bind(createBitmap(position))
    }

    private fun createBitmap(index: Int): Bitmap {
        var bitmap = bitmapGenerated[index]
        if (bitmap == null) {
            Log.d(TAG, "create a new Bitmap for page $index")
            val page = pdfRenderer.openPage(index)

            bitmap = Bitmap.createBitmap(page.width, page.height, Bitmap.Config.ARGB_8888)
            bitmap?.let {
                page.render(it, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY)
            }

            page.close()
        }

        return bitmap!!
    }

    class ViewHolder(private val view: SubsamplingScaleImageView) : RecyclerView.ViewHolder(view) {
        fun bind(bitmap: Bitmap) {
            view.setImage(ImageSource.bitmap(bitmap))
            view.isZoomEnabled = true
        }
    }
}

bitmap!! can create a nullptr