Android App crashes when switching fragments mutliply times

Question

as you can already see from the title my app is crashing when I am switching between fragments / tabs. I have 3 tabs, if I switch between the first and the second one multiply times nothing happens, if I do the same with the third and second or third to second and to the first one the app crashes.

If more than the following code and exception is required just tell me.

Exception:

E/AndroidRuntime:
FATAL EXCEPTION: main
Process: de.noel.networkscanner,
PID: 8493
java.lang.IllegalArgumentException:
Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, Parameter view
at de.noel.networkscanner.ui.main.FragmentDownlaod$registerListener$1.onItemSelected(Unknown
Source:7)
at android.widget.AdapterView.fireOnSelected(AdapterView.java:1366)
at android.widget.AdapterView.dispatchOnItemSelected(AdapterView.java:1355)
at android.widget.AdapterView.selectionChanged(AdapterView.java:1339)
at android.widget.AdapterView.checkSelectionChanged(AdapterView.java:1530)
at android.widget.AdapterView.handleDataChanged(AdapterView.java:1506)
at android.widget.AbsSpinner.onMeasure(AbsSpinner.java:191)
at android.widget.Spinner.onMeasure(Spinner.java:631)
at androidx.appcompat.widget.AppCompatSpinner.onMeasure(AppCompatSpinner.java:428)
at android.view.View.measure(View.java:24966)
at android.widget.GridLayout.measureChildWithMargins2(GridLayout.java:1018)
at android.widget.GridLayout.measureChildrenWithMargins(GridLayout.java:1028)
at android.widget.GridLayout.onMeasure(GridLayout.java:1067)
at android.view.View.measure(View.java:24966)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:978)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
at android.view.View.measure(View.java:24966)
at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1638)
at android.view.View.measure(View.java:24966)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:978)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
at android.view.View.measure(View.java:24966)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:7139)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:143)
at android.view.View.measure(View.java:24966)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:7139)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:825)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
at android.view.View.measure(View.java:24966)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:7139)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at android.view.View.measure(View.java:24966)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:7139)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:825)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
at android.view.View.measure(View.java:24966)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:7139)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at com.android.internal.policy.DecorView.onMeasure(DecorView.java:992)
at android.view.View.measure(View.java:24966)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3301)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:2028)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2330)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1888)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:8511)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:949)
at android.view.Choreographer.doCallbacks(Choreographer.java:761)
at android.view.Choreographer.doFrame(Choreographer.java:696)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:935)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7050)
at java.lang.reflect.Method.invoke(Native
Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)

My fragment classes:

1. fragment / tab (this is not the whole class):

class FragmentDownlaod : Fragment {
    constructor() : super()

    private lateinit var myView: View
    private var sizeInBit = true
    private var speedInBit = true
    private var selectedSizeForDownloadSize = DataSizeNumbers.KILO


    private var selectedSizeForDownloadSpeed = DataSizeNumbers.KILO

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        updateSpinnerItems()
        this.speedSpinner.setSelection(1)
        this.sizeSpinner.setSelection(1)
        registerListener()
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        this.myView = inflater.inflate(R.layout.download_fragement, container, false)
        return this.myView
    }

2. fragment / tab (this is not the full class):

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        this.sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this.context)


        val activity = this.activity
        if(activity != null) {

            val display: Display = activity.windowManager.defaultDisplay
            val size = Point()
            display.getSize(size)
            val width = size.x


            this.bitmap = Bitmap.createBitmap(width, width, Bitmap.Config.ARGB_8888)
        this.shapeDrawable = ShapeDrawable()


        }
    }

3. fragment / tab:

    class FragmentConverter : Fragment { constructor() : super() private lateinit var myView: View override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { this.myView = inflater.inflate(R.layout.converter_fragement, container, false) return this.myView }

EDIT: added registerListener method:

private fun registerListener() {

        textEditSize.addTextChangedListener(TextChangeWatcher(textEditSize))
        textEditSpeed.addTextChangedListener(TextChangeWatcher(textEditSpeed))

        sizeSpinner.onItemSelectedListener = (object : AdapterView.OnItemSelectedListener {
            override fun onItemSelected(
                parent: AdapterView<*>,
                view: View,
                position: Int,
                id: Long
            ) {
                when (position) {
                    0 -> selectedSizeForDownloadSize = DataSizeNumbers.BIT
                    1 -> selectedSizeForDownloadSize = DataSizeNumbers.KILO
                    2 -> selectedSizeForDownloadSize = DataSizeNumbers.MEGA
                    3 -> selectedSizeForDownloadSize = DataSizeNumbers.GIGA
                    4 -> selectedSizeForDownloadSize = DataSizeNumbers.TERRA
                }
            }

            override fun onNothingSelected(parent: AdapterView<*>) {
                selectedSizeForDownloadSize = DataSizeNumbers.KILO
            }
        })

        speedSpinner.onItemSelectedListener = (object : AdapterView.OnItemSelectedListener {
            override fun onItemSelected(
                parent: AdapterView<*>,
                view: View,
                position: Int,
                id: Long
            ) {
                when (position) {
                    0 -> selectedSizeForDownloadSpeed = DataSizeNumbers.BIT
                    1 -> selectedSizeForDownloadSpeed = DataSizeNumbers.KILO
                    2 -> selectedSizeForDownloadSpeed = DataSizeNumbers.MEGA
                    3 -> selectedSizeForDownloadSpeed = DataSizeNumbers.GIGA
                    4 -> selectedSizeForDownloadSpeed = DataSizeNumbers.TERRA
                }
            }

            override fun onNothingSelected(parent: AdapterView<*>) {
                selectedSizeForDownloadSpeed = DataSizeNumbers.KILO
            }
        })

        calcBtn.setOnClickListener {


            if (editTextIncludesNumbers(textEditSize)) {

                if (editTextIncludesNumbers(textEditSpeed)) {


                    speedResultView.text = Calculator.calculateTime(
                        getNumberFromField(textEditSize),
                        selectedSizeForDownloadSize,
                        sizeInBit,
                        getNumberFromField((textEditSpeed)),
                        selectedSizeForDownloadSpeed,
                        speedInBit
                    )

                } else {
                    speedResultView.text = getString(R.string.noCorrectNumber_speed)
                }

            } else {
                Log.d("edit", "is empty -> ${getString(R.string.noCorrectNumber_size)}")
                speedResultView.text = getString(R.string.noCorrectNumber_size)
            }


        }
        sizeBitByteSwitcher.setOnCheckedChangeListener { _, _ ->
            sizeInBit = !sizeInBit
            updateSpinnerItems()
        }

        speedBitByteSwitcher.setOnCheckedChangeListener { _, _ ->
            speedInBit = !speedInBit
            updateSpinnerItems()
        }
    }  

Answer 1

Change the view: View in your OnItemSelectedListener s to nullable view: View? .

It's a framework java interface with unspecified nullability so the kotlin compiler allows you to implement the interface with a different nonnull type. However at runtime the function can be called with a null argument, causing this crash when kotlin runtime checks your nonnulls are in fact not null.