How to upload PDF file to server with Java/Kotlin?

Question

I get an error like this after successfully selecting a PDF file. Anyone know how to solve it?

2021-06-09 09:02:17.638 7277-7277/team.getherfolg.capstone E/AndroidRuntime: FATAL EXCEPTION: main
    Process: team.getherfolg.capstone, PID: 7277
    java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=1, result=-1, data=Intent { dat=content://com.android.providers.downloads.documents/document/3964 flg=0x1 }} to activity {team.getherfolg.capstone/team.getherfolg.capstone.ui.pdfpreview.PDFPreviewActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.deliverResults(ActivityThread.java:5237)
        at android.app.ActivityThread.handleSendResult(ActivityThread.java:5278)
        at android.app.servertransaction.ActivityResultItem.execute(ActivityResultItem.java:51)
    Caused by: java.lang.NullPointerException
        at team.getherfolg.capstone.ui.pdfpreview.PDFPreviewActivity.displayFromFile(PDFPreviewActivity.kt:172)
        at team.getherfolg.capstone.ui.pdfpreview.PDFPreviewActivity.onActivityResult(PDFPreviewActivity.kt:150)
        at android.app.Activity.dispatchActivityResult(Activity.java:8294)

This is for line code 172 (Starting from pdfView!!)

private fun displayFromFile(file: File) {
    val uri = Uri.fromFile(File(file.absolutePath))
    pdfFileName = getFileName(uri)

    pdfView!!.fromFile(file)
        .defaultPage(pageNumber)
        .onPageChange(this)
        .enableAnnotationRendering(true)
        .onLoad(this)
        .scrollHandle(DefaultScrollHandle(this))
        .spacing(10)
        .onPageError(this)
        .load()
}

This is for line code 150 (Starting from displayFromFile(file))

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)

        if (requestCode == FILE_PICKER_REQUEST_CODE && resultCode == RESULT_OK && data != null) {
            val path: String = data.data.toString()
            val file = File(path)

            displayFromFile(file)

            Log.d("Path: ", path)
            pdfPath = path
            Toast.makeText(this, "Picked file: $path", Toast.LENGTH_LONG).show()
        }
    }