How to fix Parameter specified as non-null is null on rotating screen in a Fragment while creating a Spinner in Kotlin?

Question

Hello everyone in my fragment, i have added a spinner in my fragment and little bit of code to calculate moles but when i rotate screen it crashes and throw an error by i don't know why it crashes , When try to check for Non-null object i don't What that is the cause of error , Here is the error :

java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull

Here is my first fragment file :

private var mListener: OnFragmentInteractionListener? = null


override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    // Inflate the layout for this fragment
    return inflater!!.inflate(R.layout.fragment_first, container, false)
}

override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    //activity.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT)
    /*Find the id of spinner*/
    val spinner = lol

    /*set an adapter with strings array*/
    spinner.adapter = ArrayAdapter(activity, R.layout.support_simple_spinner_dropdown_item, resources.getStringArray(R.array.atoms)) as SpinnerAdapter

    /*set click listener*/
    spinner.onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
        override fun onItemSelected(parent: AdapterView<*>, view: View, position: Int, id: Long) {
            val num = when (spinner.selectedItem.toString()) {
                "H" -> editText.setText("1")
                "He" -> editText.setText("4")
                "C" -> editText.setText("12")
                "O" -> editText.setText("16")
                else -> editText.setText("")
            }
        }

        override fun onNothingSelected(parent: AdapterView<*>) {
            /*Do something if nothing selected*/
        }



    }

    button.setOnClickListener {
        if (
        editText2.text.toString().length > 0 &&
                editText.text.toString().length > 0) {
            val num2 = editText.text.toString().toDouble()
            val num1 = editText2.text.toString().toDouble()
            val num = num1/num2
            textView.setText("$num moles")
        }
        else {
            textView.setText("Please Enter a correct value")
        }
    }
}

Here is my fragment_first.xml file :

<android.support.constraint.ConstraintLayout
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <TextView
        android:id="@+id/textView"
        android:layout_width="294dp"
        android:layout_height="80dp"
        android:layout_weight="1"
        android:text="Amswer : No of Moles"
        android:textAlignment="center"
        android:textSize="20sp"
        app:layout_anchorGravity="right|top"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.556" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="143dp"
        android:layout_height="46dp"
        android:layout_weight="1"
        android:text="Amount in Grams "
        android:textAlignment="center"
        android:textSize="20sp"
        app:layout_anchorGravity="left|bottom"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.066"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.305" />

    <Spinner
        android:id="@+id/lol"
        android:layout_width="145dp"
        android:layout_height="57dp"

        app:layout_anchorGravity="left|top"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.07"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.136" />

    <Button
        android:id="@+id/button"
        android:layout_width="138dp"
        android:layout_height="43dp"
        android:text="Calculate"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.788" />

    <EditText
        android:id="@+id/editText"
        android:layout_width="148dp"
        android:layout_height="57dp"
        android:ems="10"
        android:hint="Gram"
        android:inputType="numberDecimal|number"
        app:layout_anchorGravity="bottom|center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.762"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.137" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="148dp"
        android:layout_height="57dp"
        android:ems="10"
        android:hint="Gram"
        android:inputType="numberDecimal|number"
        app:layout_anchorGravity="center_horizontal|center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.716"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.288" />

</android.support.constraint.ConstraintLayout>

Answer 1

Add ? in

view = View?

Here is code

spinner.onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
        override fun onItemSelected(parent: AdapterView<*>, view: View?, position: Int, id: Long) {
            val num = when (spinner.selectedItem.toString()) {
                "H" -> editText.setText("1")
                "He" -> editText.setText("4")
                "C" -> editText.setText("12")
                "O" -> editText.setText("16")
                else -> editText.setText("")
            }
        }